Fix a fatal on the commit author list

Summary: I think we only hit this because I mucked around with the database to
recover from the runaway parse of the Diviner repository (now prevented by
D1253), but be more robust against missing data in this interface.

Test Plan: After applying this patch, no longer received a fatal on the commit
history page for users linked to nonexistant/bogus commits.

Reviewers: jack, btrahan, jungejason, aran

Reviewed By: aran

CC: aran

Maniphest Tasks: T701

Differential Revision: https://secure.phabricator.com/D1264