Prevent crash caused by empty array when rendering Pholio mocks

Summary: This was causing a crash according to a conversation with @rfergu in IRC. The issue is that if $images_to_show is empty, then $image never gets set, and it attempts to call getID on a null / unset variable.

Test Plan: @rfergu confirmed this fixed the crash.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, rfergu, epriestley

Differential Revision: https://secure.phabricator.com/D10073