Ignore empty tasks when unfreezing recents list after timeout

- The top task can sometimes be a root that doesn't have any
  children (ie. split root), which can lead to an empty
  task being in the recents list

Bug: 275425871
Test: atest SwitchBetweenSplitPairs
Change-Id: Ia0bd7a08870e8cbd4849f79506fd8db9a1d18d65