Fix github trigger when submitting a webhook without a head_commit

Fixes #2125

test/test_prepare_trigger.py

@@ -443,5 +443,11 @@ class TestPrepareTrigger(unittest.TestCase):
 
     self.assertSchema('github_webhook_deletedcommit', expected, gh_webhook, lookup_user=lookup_user)
 
+  def test_github_webhook_known_issue(self):
+    def lookup_user(_):
+      return None
+
+    self.assertSkipped('github_webhook_knownissue', gh_webhook, lookup_user=lookup_user)
+
 if __name__ == '__main__':
   unittest.main()