Do not attempt auto-enter-pip if there is no valid leash

No reproduce path for the bug being reported, this fix is to ensure we
do not start the auto-enter-pip animation with invalid leash at the
first place.

Bug: 234796719
Test: N/A for the bug, verify the normal auto-enter-pip still works
Change-Id: I6f2164a322877f1b0213a1ee09eb476090300c7f