Major Description
As discussed here [1] the NonExistingResource should also be a SyntheticResource because it is not actually reflected with a real resource but is returned from the ResourceResolver.resolve(HttpServletRequest) method when the request cannot be resolved to a real resource.
In addition a new helper method should be added to the JcrResourceUtil:
// return true if resource is a synthetic resource
public static boolean isSyntheticResource(Resource resource);