Prevent confusion with empty vs absent model specification

[stefanoborini]

At the moment, it is my understanding that an empty models: specification means that the CUDS Item is associated to any model. This is a bad idea, because we have to treat the special case of empty as "all" when we do a check for inclusivity (the in operator).

The correct solution is to leave models unspecified if the Item is valid for any model, and consider the empty option logically, as an Item that is not valid for any model.