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

@@ -14,7 +14,10 @@ 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'