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