Remove NULLS from constants.py
    
    In the parameters system, we have been checking for a positive list of values
    which get converted to None. The problem is that this method can in some
    cases throw warnings when type coercion doesn't work (particularly, string
    to unicode). Instead, any values that evaluate to False that are neither
    numeric nor boolean should be converted to None.
    
    Reviewed-By: Jan Pazdziora <jpazdziora@redhat.com>