Trebuchet: Fix TaskbarModelCallbacksFactory instantiation

Overrides.getObject() calls the constructor with context if the overlay
has a valid class, if empty it uses the constructor without args.
Considering this class is part of quickstep, we should support both
cases, since this is the fallback class anyway

Change-Id: Ic412a2a3efbe1d3f8b9c02a58635c2821eabd54d