Filter contact sub-type by "is not one of" fails
When filtering by contact sub-type, "is not one of" excludes all those that do not have a sub-type.
Noted previously here: #15 (comment 25491)
For example, filtering by "is not one of" 'Student' should just exclude those who do have a sub-type of Student. Currently, it also excludes all those who have a sub-type of Student, and also all those with no sub-type.