Skip bitbucket pushes without any commits

Fixes 178220183/

test/test_prepare_trigger.py

@@ -511,5 +511,9 @@ class TestPrepareTrigger(unittest.TestCase):
 
     self.assertSkipped('github_webhook_knownissue', gh_webhook, lookup_user=lookup_user)
 
+
+  def test_bitbucket_webhook_known_issue(self):
+    self.assertSkipped('bitbucket_knownissue', bb_webhook)
+
 if __name__ == '__main__':
   unittest.main()