Fix Gitlab trigger payload bug when commits is empty

Gitlab will occasionally send trigger payloads with an empty commit list (and a null checkout_ha) for branches that have been deleted. Properly handle that case.

test/test_prepare_trigger.py

@@ -8,8 +8,20 @@ from buildtrigger.bitbuckethandler import get_transformed_webhook_payload as bb_
 from buildtrigger.bitbuckethandler import get_transformed_commit_info as bb_commit
 from buildtrigger.githubhandler import get_transformed_webhook_payload as gh_webhook
 from buildtrigger.gitlabhandler import get_transformed_webhook_payload as gl_webhook
+from buildtrigger.triggerutil import SkipRequestException
 
 class TestPrepareTrigger(unittest.TestCase):
+  def assertSkipped(self, filename, processor, *args, **kwargs):
+    with open('test/triggerjson/%s.json' % filename) as f:
+      payload = json.loads(f.read())
+
+    nargs = [payload]
+    nargs.extend(args)
+
+    with self.assertRaises(SkipRequestException):
+      processor(*nargs, **kwargs)
+
+
   def assertSchema(self, filename, expected, processor, *args, **kwargs):
     with open('test/triggerjson/%s.json' % filename) as f:
       payload = json.loads(f.read())
@@ -349,6 +361,9 @@ class TestPrepareTrigger(unittest.TestCase):
     self.assertSchema('github_webhook_missing', expected, gh_webhook)
 
 
+  def test_gitlab_webhook_nocommit_payload(self):
+    self.assertSkipped('gitlab_webhook_nocommit', gl_webhook)
+
   def test_gitlab_webhook_payload_with_lookup(self):
     expected = {
       'commit': u'fb88379ee45de28a0a4590fddcbd8eff8b36026e',