Fix appearance of "Differential Revision" on edit interfaces

Summary: The recent change to the field causes us to render "http://junk.com/D"
in some cases, just null the field if there's no data.

Test Plan: Ran "arc diff --create".

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1321