Fix differential.createrevision conduit call

Summary:
This introduces some boundary checking for
PhabricatorOwnersOwner::loadAffiliatedUserPHIDs() if it gets passed an empty
array, which happened when I ran arc diff and it called
differential.createrevision.

Test Plan: ran arc diff

Reviewers: epriestley, meitros, jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2112