Remove empty desktop tile

Do not show the desktop tile in overview when there are no apps on the
desktop.

Bug: 270399069
Test: have desktop mode proto 2 enabled, launch an app in fullscreen,
  open overview, observe no desktop tile
Test: move an app to desktop, be on desktop, open overview, observe
  overview is opened with desktop tile focused
Test: have an app on desktop, go home and launch an app in fullscreen,
  open overview, observe fullscreen app is focused, desktop is peeking
  in from right
Test: have an app on desktop and a fullscreen app in overview, open
  desktop from overview, close the desktop app, open overview, observe
  desktop tile is not shown
Change-Id: Ia8657d5b260043a630f32b35f2560ea93273d421