Recover from null authors in commit parsing

Summary:
Fixes T5445. Some import tools and other unusual situations can leave repositories with commits that don't have authors. This fails on insert.

Instead, explicitly cast the value to a string.

Test Plan: I didn't build a local repro, but see task/GitHub.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5445

Differential Revision: https://secure.phabricator.com/D9684

src/applications/repository/worker/commitmessageparser/PhabricatorRepositoryCommitMessageParserWorker.php

@@ -17,7 +17,7 @@ abstract class PhabricatorRepositoryCommitMessageParserWorker
       $data = new PhabricatorRepositoryCommitData();
     }
     $data->setCommitID($commit->getID());
-    $data->setAuthorName($author);
+    $data->setAuthorName((string)$author);
     $data->setCommitDetail(
       'authorPHID',
       $this->resolveUserPHID($commit, $author));