Revert "Revert "Don't filter out groups if summary is re-inflating""

Add an extra null check to handle pruned group summaries.

Fixes: 249150028
Test: atest PreparationCoordinatorTest
Test: manual
  0. Enable ADB debugging and plug in device to USB port
  1. Go to lockscreen, have "USB debugging connected" as *only*
  notification
  2. Turn screen off w/ power button
  Observe: no clock flicker

Change-Id: I5ac35be2cb40d001aa294dfe9f0bc88dca8ff62e