Differential - fix "transaction has no effect error" on diff create

Summary: Fixes T6658.

Test Plan: made a diff with no repository and default policy and it worked!

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: Korvin, epriestley

Maniphest Tasks: T6658

Differential Revision: https://secure.phabricator.com/D10910