fix(issues/event): init ShortUser only if actor is defined

Author: caugner

src/github3/issues/event.py

@@ -60,7 +60,8 @@ class IssueEvent(GitHubCore):
 
     def _update_attributes(self, event):
         self._api = event["url"]
-        self.actor = users.ShortUser(event["actor"], self)
+        if event["actor"]:
+            self.actor = users.ShortUser(event["actor"], self)
         self.commit_id = event["commit_id"]
         self.commit_url = event["commit_url"]
         self.created_at = self._strptime(event["created_at"])