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 https://sentry.io/coreos/backend-production/issues/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,

buildtrigger/test/githubmock.py

@@ -53,7 +53,12 @@ def get_mock_github():
     repo_mock.full_name = '%s/%s' % (namespace, name)
     repo_mock.name = name
     repo_mock.description = 'some %s repo' % (name)
-    repo_mock.pushed_at = datetime.utcfromtimestamp(0)
+
+    if name != 'anotherrepo':
+      repo_mock.pushed_at = datetime.utcfromtimestamp(0)
+    else:
+      repo_mock.pushed_at = None
+
     repo_mock.html_url = 'https://bitbucket.org/%s/%s' % (namespace, name)
     repo_mock.private = name == 'somerepo'
     repo_mock.permissions = Mock()