Fix wallpaper NPE caused by user switching before IWallpaperService connection

- car side is doing quick user switching during boot-up and that leads into
  wallpaper user switching before the service connection.
  user 0 start, user 0 unlock, user switch to user 10, boot complete, user 10 unlock

Bug: 129569939
Test: Try user switching flow in car env and confirm no crash
Change-Id: I764f68377ddf8b581f8386a236aa5c24a96368dd

services/core/java/com/android/server/wallpaper/WallpaperManagerService.java

@@ -2671,7 +2671,10 @@ public class WallpaperManagerService extends IWallpaperManager.Stub
                 wallpaper.connection.mReply = null;
             }
             try {
-                wallpaper.connection.mService.detach();
+                // It can be null if user switching happens before service connection.
+                if (wallpaper.connection.mService != null) {
+                    wallpaper.connection.mService.detach();
+                }
             } catch (RemoteException e) {
                 Slog.w(TAG, "Failed detaching wallpaper service ", e);
             }