Enforce that required attributes can't be set to None in CRUD Update
    
    The `required` parameter attribute didn't distinguish between cases
    where the parameter is not given and all, and where the parameter is
    given but empty. The case of updating a required attribute couldn't
    be validated properly, because when it is given but empty, validators
    don't run.
    This patch introduces a new flag, 'nonempty', that specifies the
    parameter can be missing (if not required), but it can't be None.
    This flag gets added automatically to required parameters in CRUD
    Update.