[PiP2] Start with non-null pip params

Apps and certain tests actually send empty
PiP params with direct enterPictureInPictureMode()
API calls, which ends up being converted to null when
doing the fillTaskInfo() for TransitionInfo.Changes in Core.

So we need to avoid NPEs in Shell when resolving these transitions.
We do this by starting out with an empty pip params if a null one is
provided through a Transitions PiP target.

Bug: 379390590
Flag: com.android.wm.shell.enable_pip2
Test: atest PinnedStackTests#testNonTappablePipActivity
Change-Id: I165d565f6a7e33ec3c0f5aa53d6fd1cebc2cdac7