Return the default snap bounds if there is no active desk

DesktopTilingDecorViewModel.get{Right,Left}SnapBoundsIfTiled can be
called when a window drag starts from fullscreen.
It should return the default snap bounds rather than an empty bounds.

Bug: 421068243
Test: manual - follow the step in the bug
Test: atest WMShellUnitTests
Flag: EXEMPT small bug fix
Change-Id: I3edff93694bc752f376724d50b102a5a02dbae59