github_webhook: Use author's full name if username is not available.

In cases where the webhook payload doesn't have the username for the
author of a particular commit (this can happen if the author doesn't
have a GitHub account or the author's email is not associated with
their GitHub account), we now use the author's full name to format
messages.

zerver/webhooks/github_webhook/view.py

@@ -180,7 +180,8 @@ def get_push_tags_body(payload):
 def get_push_commits_body(payload):
     # type: (Dict[str, Any]) -> Text
     commits_data = [{
-        'name': commit['author']['username'],
+        'name': (commit.get('author').get('username') or
+                 commit.get('author').get('name')),
         'sha': commit['id'],
         'url': commit['url'],
         'message': commit['message']