@Override
 public Object findNullSerializer(Annotated a) {
   JsonSerialize ann = _findAnnotation(a, JsonSerialize.class);
   if (ann != null) {
     Class<? extends JsonSerializer<?>> serClass = ann.nullsUsing();
     if (serClass != JsonSerializer.None.class) {
       return serClass;
     }
   }
   return null;
 }