Add null-check before reparenting content overlay

It is a common case that an auto-enter-pip eligible activity specifies
source rect hint and there is no content overlay being used during the
auto-enter-pip animation.

Bug: 232857181
Test: swipe auto-enter-pip eligible activity to home with shell \
      transition being turned on
Change-Id: Id31d45764e557c5ea5a7918f49e620342cbeeb7a