Annotated result of StubPlayer.getPlayerError with @Nullable

libraries/test_utils/src/main/java/androidx/media3/test/utils/StubPlayer.java

@@ -70,7 +70,7 @@ public class StubPlayer extends BasePlayer {
   }
   }
 
 
   @Override
   @Override
-  public PlaybackException getPlayerError() {
+  public @Nullable PlaybackException getPlayerError() {
     throw new UnsupportedOperationException();
     throw new UnsupportedOperationException();
   }
   }