resourceloader: Implement isKnownEmpty for user.options module

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