Fix potential NPE & clean up some logic

- I had accidentally swapped the values in removeBubblesForUser
  in one place which could lead to a crash, it's actually simpler
  to just do the removal there since it's the parentUser that was
  removed so we can remove all of those entries.
- In other places I've added a null check before performing the
  removeIf to be safe
- Added a test checking bad input

Bug: 232847471
Test: atest BubbleVolatileRepositoryTest
Change-Id: I10f5b24e591db62edbb63ad40e588ba19ab621c1