Check for $BUILDKITE_PULL_REQUEST = false when uploading sarif file.

[wzieba]

If build is not associated with a pull request, Buildkite puts value false for $BUILDKITE_PULL_REQUEST
Initially, I thought it just doesn't create such variable.

Proof can be found here: https://buildkite.com/automattic/pocket-casts-android/builds/8308#0191e617-a1e2-4ae6-a7bb-4ef3dda51370

Because of this bug, after merge to trunk in Pocket Casts we can see:

{
  | [2024-09-12T12:07:17Z]   "message": "ref 'refs/pull/false/head' not found in this repository",
  | [2024-09-12T12:07:17Z]   "documentation_url": "https://docs.github.com/rest/code-scanning/code-scanning#upload-an-analysis-as-sarif-data",
  | [2024-09-12T12:07:17Z]   "status": "404"
  | [2024-09-12T12:07:17Z] 
}

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• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[AliSoftware]

👍