Add null check to onTransitionMerged

Checks to see if mTransitionPausingRelayout is null before comparing it
to the merged transition IBinder.

Bug: 276973868
Test: Manual Testing
Change-Id: I772a2dfa4c29c0a78b09c2c1c7f0a5a790795604

libs/WindowManager/Shell/src/com/android/wm/shell/windowdecor/DesktopModeWindowDecorViewModel.java

@@ -197,7 +197,7 @@ public class DesktopModeWindowDecorViewModel implements WindowDecorViewModel {
 
     @Override
     public void onTransitionMerged(@NonNull IBinder merged, @NonNull IBinder playing) {
-        if (mTransitionPausingRelayout.equals(merged)) {
+        if (merged.equals(mTransitionPausingRelayout)) {
             mTransitionPausingRelayout = playing;
         }
     }