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