Trebuchet: Fix Taskbar{Model,View}CallbacksFactory 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

Co-authored-by: Aaron Kling <webgeek1234@gmail.com>
Fixes: I016d4e4e6a096ca4f5bd65fcda1bf24a444459f4
Change-Id: Ic412a2a3efbe1d3f8b9c02a58635c2821eabd54d