Ignore recents transition if there are no closing tasks

- In the case where Launcher calls startRecentsTransition while there
  are no other visible tasks, we should not be continuing with the
  transition as there are no tasks for Launcher to control.  This was
  previously handled in RecentsAnimationController in legacy
  transitions, but the safer fix is to ignore it on the Launcher
  side for this release.

Bug: 289175232
Test: Manually trigger empty targets and verify no issues
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:6f7e15ff644bac2f11ecb779030255efebc18885)
Merged-In: I3657c000cbc8c14c9ac989c2a57715515c96edb6
Change-Id: I3657c000cbc8c14c9ac989c2a57715515c96edb6