Fix NPE from startNewTransition

With shell transition, the token can be null from
WindowOrganizerController#startNewTransition.
E.g. entering split screen.

Bug: 258913831
Test: atest TransitionTests
Change-Id: I77e85c4a340a88056c8188f5da99cf57bf52197e

services/core/java/com/android/server/wm/Transition.java

@@ -228,7 +228,8 @@ class Transition implements BLASTSyncEngine.TransactionReadyListener {
     }
 
     @Nullable
-    static Transition fromBinder(@NonNull IBinder token) {
+    static Transition fromBinder(@Nullable IBinder token) {
+        if (token == null) return null;
         try {
             return ((Token) token).mTransition.get();
         } catch (ClassCastException e) {