Bug 1789398 - ensure task["metadata"]["owner"] is not empty on github…

…-push decision tasks

Treeherder requires this in its ingestion pipeline, so if the pusher
doesn't have an email then use their name instead.

.taskcluster.yml

@@ -16,7 +16,10 @@ tasks:
                   then: '[email protected]'   # It must match what's in bors.toml
                   else:
                       $if: 'tasks_for == "github-push"'
-                      then: '${event.pusher.email}'
+                      then:
+                          $if: 'event.pusher.email'
+                          then: '${event.pusher.email}'
+                          else: '${event.pusher.name}@users.noreply.github.com'
                       else:
                           $if: 'tasks_for == "github-pull-request"'
                           then: '${event.pull_request.user.login}@users.noreply.github.com'