Add check for None returned from custom resolvers.

See #3 (closed). I noticed that, in the event a resolver function for a field returns None, the FilterMixin would return all model instances instead of no model instances. This affects the ModelFilterField and the ModelFilterPageField, as they both inherit from FilterMixin. I added a check in filter_resolver() for FilterMixin for None, and made it so that it would pass the underlying FilterSet an empty qs instead of None.