Handling cases with a null bitmap.

- This can happen either if an app creates an ActivityOptions without a
  thumbnail, or if the call to create a hardware bitmap fails for any
  reason. Just ignore the thumbnail for the transition in this case.

Bug: 62296016
Test: Have not been able to reproduce, but this is just a logical change
Change-Id: I30776b651df1f42118fe1d317fa4817261a6e977