resourceloader: Implement isKnownEmpty for user.options module
This allows ResourceLoaderClientHtml to output state=ready instead
of state=loading with a no-op mw.loader.implement() call being embedded.
Test Plan:
* View source on page view when logged-out.
* Before:
- loader.state({"user.options":"loading"})
- loader.implement("user.options@..",function(){ .. user.options.state([]) })
* After:
- loader.state({"user.options":"ready"})
Bug: T176159
Change-Id: I18f76eaa960da9b0ca77f28e10f768587447a838