GitHub sometimes sends a pull event without a sender

[kvanbere]

Operating system or device, package version, compiler version:
Affects all.

Issue description:
In the push event, we were expecting the sender of the push event to always be a known User, but in some cases the User can be null.
Reported by @roberth.

Steps to reproduce:
See this example fixture:

{
  "ref": "refs/heads/gh-readonly-queue/main/pr-3072-287d30540ac5a1acc57d0bdc138fa81dee315f48",
  "before": "a6bd77bbe77ee6ae8f9686621b192d70fcf83e2b",
  "after": "0000000000000000000000000000000000000000",
  "repository": {
    "id": 353322834,
    "node_id": "MDEwOJllG9zcXaRcvknzNYyTzM4OUQD=",
    "name": "infra-core",
    "full_name": "some-organization/infra-core",
    "private": true,
    "owner": {
      "name": "some-organization",
      "email": null,
      "login": "some-organization",
      "id": 52709322,
      "node_id": "MDEyO9ky2ZuaXpFdhlGvjbzNzczMTO3Y",
      "avatar_url": "https://avatars.githubusercontent.com/u/52709322?v=4",
      "gravatar_id": "",
      "url": "https://api.github.com/users/some-organization",
      "html_url": "https://github.com/some-organization",
      "followers_url": "https://api.github.com/users/some-organization/followers",
      "following_url": "https://api.github.com/users/some-organization/following{/other_user}",
      "gists_url": "https://api.github.com/users/some-organization/gists{/gist_id}",
      "starred_url": "https://api.github.com/users/some-organization/starred{/owner}{/repo}",
      "subscriptions_url": "https://api.github.com/users/some-organization/subscriptions",
      "organizations_url": "https://api.github.com/users/some-organization/orgs",
      "repos_url": "https://api.github.com/users/some-organization/repos",
      "events_url": "https://api.github.com/users/some-organization/events{/privacy}",
      "received_events_url": "https://api.github.com/users/some-organization/received_events",
      "type": "Organization",
      "site_admin": false
    },
    "html_url": "https://github.com/some-organization/infra-core",
    "description": "Homo sapiens non urinat in ventum",
    "fork": false,
    "url": "https://github.com/some-organization/infra-core",
    "forks_url": "https://api.github.com/repos/some-organization/infra-core/forks",
    "keys_url": "https://api.github.com/repos/some-organization/infra-core/keys{/key_id}",
    "collaborators_url": "https://api.github.com/repos/some-organization/infra-core/collaborators{/collaborator}",
    "teams_url": "https://api.github.com/repos/some-organization/infra-core/teams",
    "hooks_url": "https://api.github.com/repos/some-organization/infra-core/hooks",
    "issue_events_url": "https://api.github.com/repos/some-organization/infra-core/issues/events{/number}",
    "events_url": "https://api.github.com/repos/some-organization/infra-core/events",
    "assignees_url": "https://api.github.com/repos/some-organization/infra-core/assignees{/user}",
    "branches_url": "https://api.github.com/repos/some-organization/infra-core/branches{/branch}",
    "tags_url": "https://api.github.com/repos/some-organization/infra-core/tags",
    "blobs_url": "https://api.github.com/repos/some-organization/infra-core/git/blobs{/sha}",
    "git_tags_url": "https://api.github.com/repos/some-organization/infra-core/git/tags{/sha}",
    "git_refs_url": "https://api.github.com/repos/some-organization/infra-core/git/refs{/sha}",
    "trees_url": "https://api.github.com/repos/some-organization/infra-core/git/trees{/sha}",
    "statuses_url": "https://api.github.com/repos/some-organization/infra-core/statuses/{sha}",
    "languages_url": "https://api.github.com/repos/some-organization/infra-core/languages",
    "stargazers_url": "https://api.github.com/repos/some-organization/infra-core/stargazers",
    "contributors_url": "https://api.github.com/repos/some-organization/infra-core/contributors",
    "subscribers_url": "https://api.github.com/repos/some-organization/infra-core/subscribers",
    "subscription_url": "https://api.github.com/repos/some-organization/infra-core/subscription",
    "commits_url": "https://api.github.com/repos/some-organization/infra-core/commits{/sha}",
    "git_commits_url": "https://api.github.com/repos/some-organization/infra-core/git/commits{/sha}",
    "comments_url": "https://api.github.com/repos/some-organization/infra-core/comments{/number}",
    "issue_comment_url": "https://api.github.com/repos/some-organization/infra-core/issues/comments{/number}",
    "contents_url": "https://api.github.com/repos/some-organization/infra-core/contents/{+path}",
    "compare_url": "https://api.github.com/repos/some-organization/infra-core/compare/{base}...{head}",
    "merges_url": "https://api.github.com/repos/some-organization/infra-core/merges",
    "archive_url": "https://api.github.com/repos/some-organization/infra-core/{archive_format}{/ref}",
    "downloads_url": "https://api.github.com/repos/some-organization/infra-core/downloads",
    "issues_url": "https://api.github.com/repos/some-organization/infra-core/issues{/number}",
    "pulls_url": "https://api.github.com/repos/some-organization/infra-core/pulls{/number}",
    "milestones_url": "https://api.github.com/repos/some-organization/infra-core/milestones{/number}",
    "notifications_url": "https://api.github.com/repos/some-organization/infra-core/notifications{?since,all,participating}",
    "labels_url": "https://api.github.com/repos/some-organization/infra-core/labels{/name}",
    "releases_url": "https://api.github.com/repos/some-organization/infra-core/releases{/id}",
    "deployments_url": "https://api.github.com/repos/some-organization/infra-core/deployments",
    "created_at": 1618165822,
    "updated_at": "2022-11-07T17:04:28Z",
    "pushed_at": 1670262117,
    "git_url": "git://github.com/some-organization/infra-core.git",
    "ssh_url": "[email protected]:some-organization/infra-core.git",
    "clone_url": "https://github.com/some-organization/infra-core.git",
    "svn_url": "https://github.com/some-organization/infra-core",
    "homepage": "https://github.com/orgs/some-organization/projects/7",
    "size": 23932,
    "stargazers_count": 4,
    "watchers_count": 4,
    "language": "Brainfuck",
    "has_issues": true,
    "has_projects": true,
    "has_downloads": true,
    "has_wiki": true,
    "has_pages": false,
    "has_discussions": true,
    "forks_count": 1,
    "mirror_url": null,
    "archived": false,
    "disabled": false,
    "open_issues_count": 130,
    "license": {
      "key": "other",
      "name": "Other",
      "spdx_id": "NOASSERTION",
      "url": null,
      "node_id": "MDc6GTjlW5ZZzAT="
    },
    "allow_forking": false,
    "is_template": false,
    "web_commit_signoff_required": false,
    "topics": [

    ],
    "visibility": "private",
    "forks": 1,
    "open_issues": 130,
    "watchers": 4,
    "default_branch": "main",
    "stargazers": 4,
    "master_branch": "main",
    "organization": "some-organization"
  },
  "pusher": {
    "name": "none"
  },
  "organization": {
    "login": "some-organization",
    "id": 52709322,
    "node_id": "MDEyO9kZyFua2pXhGlvdjbzNcMzOTYz3",
    "url": "https://api.github.com/orgs/some-organization",
    "repos_url": "https://api.github.com/orgs/some-organization/repos",
    "events_url": "https://api.github.com/orgs/some-organization/events",
    "hooks_url": "https://api.github.com/orgs/some-organization/hooks",
    "issues_url": "https://api.github.com/orgs/some-organization/issues",
    "members_url": "https://api.github.com/orgs/some-organization/members{/member}",
    "public_members_url": "https://api.github.com/orgs/some-organization/public_members{/member}",
    "avatar_url": "https://avatars.githubusercontent.com/u/52709322?v=4",
    "description": "Plurality Media"
  },
  "installation": {
    "id": 17816238,
    "node_id": "MDIzklOdGunVcFm0WauSW9z5GFdsGb0FW9uaTMgM3zcNjg4="
  },
  "created": false,
  "deleted": true,
  "forced": false,
  "base_ref": null,
  "compare": "https://github.com/some-organization/infra-core/compare/a6bd77bbe77e...000000000000",
  "commits": [

  ],
  "head_commit": null
}

Link to minimal example project:
N/A