Allow null identities to be attached to commit objects

Summary: I landed D19491 a little aggressively, so allow this field to be null until after the migration goes out.

Test Plan: Loaded commits without identity objects; did not get any errors.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19496