User Picker Custom Fields should have an entry "None" when searching for issues

As JRA-8631 is resolved now (User Picker Custom Fields should have drop-down-box for "Current User" etc. when searching for issues) there is another request:

While you can search for "Unassigned" for an assignee you cannot do an equal search on User Picker Custom Fields. I would expect to have something like "None" to search for.