Fix NoneType error in GH trigger when it has no pushed_at field

This appears to occur in new repositories that have not yet been pushed.

Fixes 234686591/

buildtrigger/githubhandler.py

@@ -310,7 +310,7 @@ class GithubBuildTrigger(BuildTriggerHandler):
         'name': repo.name,
         'full_name': repo.full_name,
         'description': repo.description or '',
-        'last_updated': timegm(repo.pushed_at.utctimetuple()),
+        'last_updated': timegm(repo.pushed_at.utctimetuple()) if repo.pushed_at else 0,
         'url': repo.html_url,
         'has_admin_permissions': repo.permissions.admin,
         'private': repo.private,