Some ReleephRequests don't have diffs

Summary: Not every commit requested in Releeph has to have come from a Differential revision. If you have a Releeph request for a commit without a Differential revision, you currently get this lovely error in the logs: {P824}. This diff silences the howling.

Test Plan: Load a Releeph branch page; observe log silence.

Reviewers: wez, epriestley

Reviewed By: epriestley

CC: epriestley, aran, SEJeff, Korvin

Differential Revision: https://secure.phabricator.com/D5823