Fix getBitmapCrops

Fixe two cases where getBitmapCrops could return empty Rect if the
cropHint of the wallpaper is empty, which can happen with
 - the default wallpaper
 - a wallpaper coming from forward migration after reboot

Flag: aconfig com.android.window.flags.multi_crop TEAMFOOD
Bug: 333456932
Test: manual
Change-Id: I46806cab1735f4ab26511750907d528fc448e1c6