Fix a warning in BoardLayoutEngine when no objects are being updated
Summary: Fixes T13368. Some workflows (like "Move tasks to...") execute board layout without objects to update. In these cases, we can hit a warning because objectPHIDs is not initialized to array().
Test Plan: Went through the "Move tasks to..." workflow on a workboard, no longer saw a warning when trying to iterate over an empty objectPHIDs list.
Maniphest Tasks: T13368
Differential Revision: https://secure.phabricator.com/D20701