fixed slow call user

Author: Maximilian Karl

github2pandas/utility.py

@@ -475,6 +475,8 @@ def extract_author_data_from_commit(repo, sha, users_ids, data_root_dir):
         commit = repo.get_commit(sha)
         if not commit:
             return None
+        if commit._author == github.GithubObject.NotSet:
+            return None
         return Utility.extract_user_data(commit.author, users_ids, data_root_dir)
 
     @staticmethod
@@ -510,6 +512,8 @@ def extract_committer_data_from_commit(repo, sha, users_ids, data_root_dir):
         commit = repo.get_commit(sha)
         if not commit:
             return None
+        if commit._committer == github.GithubObject.NotSet:
+            return None
         return Utility.extract_user_data(commit.committer, users_ids, data_root_dir)
 
     @staticmethod