Add empty view to desktop tile in overview

Show an empty view when there are no tasks on the desktop.

Bug: 267326725
Test: have a fullscreen task, open overview, observe empty view for
  desktop tile
Test: have a desktop task, open overview, observe desktop task thumbnail
  is shown in overview and empty view is hidden

Change-Id: Ifb6d96d42bf602e9cc9e397efc0e07e2accdf16b