Set stack bounds to null instead of empty for fullscreen

When dragging docked stack divider we evaluate how other stack
bounds should change. For non-resizeable launchers computed rect
is empty, so we should send null to resize call instead which
corresponds to fullscreen.

Change-Id: Ie005be2b9872181e31529b592211423049e4ef09
Fixes: 37752990
Test: Drag the divider with non-resizeable launcher.