Let deepCopyRectsObjectRecycling handle nulls

The input array is @NonNull. Sadly the elements of the array are not
@NonNull, which is the important part of being @NonNull. Since passing
nulls was OK in the past, we'll continue to support it.

Fix: 410803519
Bug: 410399557
Flag: com.android.window.flags.reduce_changed_exclusion_rects_msgs
Test: atest SystemGestureExclusionRectsTest
Change-Id: I0b37694332096bad50316e874927cb230e34821a