[PiP2] Add a null check to move_pip_to_pinned

Since HIERARCHY_OP_TYPE_MOVE_PIP_ACTIVITY_TO_PINNED_TASK
can be technically used wherein any token is provided for the task,
not necessarily one including a PiP activity, we should have a null
check for PiP activity.

We aren't supposed to hit a case like this in normal PiP CUJs;
instead this is relevant for Desktoop Windowing CUJs (e.g. minimize)
where this could be called directly without a pipCandidate provided
earlier through handleRequest into the handler.

Bug: 406374716
Flag: com.android.wm.shell.enable_pip2
Test: minimize to potentially PiP a non-PiPable activity in DW
Change-Id: Ia25989cc8dcfd2c0629e2185cb93f65be68e20a0