fix: ignore invalid content from GitHub in case of deleted teams

otterdog/providers/github/graphql.py

@@ -281,6 +281,9 @@ def _transform_actors(actors: list[dict[str, Any]]) -> list[str]:
         result = []
         for actor_wrapper in actors:
             actor = actor_wrapper["actor"]
+            if actor is None:
+                continue
+
             typename = actor["__typename"]
             if typename == "User":
                 login = actor["login"]