Add default empty resources for unconfigured uses.

If Rincl is used unconfigured, it will throw a ConcernNotFoundException. Yet it would be reasonable not to configure Rincl at all if all Resources access used the Optional variety with defaults, e.g.:

Rincl.getResourceI18nConcern() should be upgraded so that, if no Rincl concern is registered, to return an empty resources concern (a singleton instance) that uses EmptyResources. Note that this will be done most efficiently if Csar is updated to allow Optional<T> for Csar.getConcern(...).