fix(input): improve html5 validation for ngModelOptions

This is a minor adjustment over the recent html5 validation fix in order to make it work correctly with ngModelOptions. The gist is that `$commitViewValue` now runs the validation pipeline even if the view value remains empty in order to refresh the validity state.
@citp I couldn't understand from your comments if there is a specific use case that you think is not covered here, or if you only think that an additional refactor is required for the form validation process. An example would probably be very helpful.