Enforce unique value for invisible field

With #11953 (closed) the enforcement is not done if the field is not required but sometimes the field is not required but invisible. Thus having an invalid value is a problem without #7138 (closed).

Indeed a common pattern for fields that are useless in some states is to set a domain line:

domain=[
    If(Eval(...),
       ('field', '=', None),
       ()),
    ]

This ensures that the field is cleared with not usable.