Prevent split-screen decor animation on invalid snapshot (null)

Check if the snapshot is valid before animating the decor. This avoids a bug where `SplitScreenTransitions.onFinish` is not called, which causes the split-screen to freeze.

Bug: 434127815
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: On a foldable device (fold mode), enter split-screen and repeatedly swap the split ratio between 10:90 and 90:10.
Change-Id: Iae30083bc3f9ad4291a79f17eb02e9de2fb1b353