Fix github trigger when submitting a webhook without a head_commit

Fixes #2125

buildtrigger/githubhandler.py

@@ -29,7 +29,7 @@ GITHUB_WEBHOOK_PAYLOAD_SCHEMA = {
       'type': 'string',
     },
     'head_commit': {
-      'type': 'object',
+      'type': ['object', 'null'],
       'properties': {
         'id': {
           'type': 'string',
@@ -98,6 +98,9 @@ def get_transformed_webhook_payload(gh_payload, default_branch=None, lookup_user
 
   payload = JSONPathDict(gh_payload)
 
+  if payload['head_commit'] is None:
+    raise SkipRequestException
+
   config = SafeDictSetter()
   config['commit'] = payload['head_commit.id']
   config['ref'] = payload['ref']