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