Check if recents exists before trying to relz to it

In some situations (SUW) recents doesn't exist at the time of the
gesture. In these situations, don't try to reference it.

Bug: 205062356
Test: Dismiss AllSetActivity via swipe-up in SUW, observe no crash
Change-Id: I3f1bfd04cfa4874475cd6d033376f24cb5b192a7