Differential - fix bogus foreach(null)

Summary: Default $phids to array() and update it if getValue() has something pertinent... Fixes T6292.

Test Plan: just used the ole logic noodle on this one.

Reviewers: chad, epriestley

Reviewed By: chad, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6292

Differential Revision: https://secure.phabricator.com/D10697