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

@@ -12,7 +12,10 @@ tasks:
               then: '${tasks_for}@noreply.mozilla.org'
               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'