Fix NPE from accessing pausing tasks

- It's possible for mPausingTasks to be unset between
  startRecentsTransition() and startAnimation(), and in these cases
  canceling with a screenshot can not take the snapshots as we have no
  leashes of the tasks in the animation itself.  For now, just cancel
  the transition without the snapshots.

Fixes: 286874048
Test: Just adding an NPE check
Change-Id: Ie1315d8f6451cfb9b2ee2782c7d89db3cbb36aac