Add null check in desktopWallpaperChanges()

getToken will return null if there is no wallpaper in that display,
but transition changes in that display that have a null container would
mistakenly be identified as desktop wallpaper changes if there is
no null check first.

Flag: EXEMPT BUGFIX
Bug: 445516496
Test: atest DesksTransitionObserverTest
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:f781163c393e9e6caca05ddb0d7e7760fdd84986
Merged-In: I5311b60e1ab0147f5b7e8b99a3b190ce7945d01c
Change-Id: I5311b60e1ab0147f5b7e8b99a3b190ce7945d01c