Fix a potential NPE

I've never been able to reproduce this and the stack traces I have
of it don't match existing code, but looking at the code paths I think
this could still happen, there are a couple of places where the
mMagnetizedBubbleDraggingOut gets nulled out (e.g. on child removed)
I've tried to repro by:
 - canceling the bubbles in the middle of dragging to dismiss
 - trying to snap the bubble back and then drag it again to dismiss
   (snap back also nulls it)

I see a recent stack of it in pitot so I think it's worth adding the
null check.

Test: atest ExpandedAnimationControllerTest
Bug: 201866808
Change-Id: Ib9425a2b63410c95fddaaea59761dfd18557ed54