Fix reporting of "not checked" for check boxes with no other states in browse mode. (#10781)
When determining whether the caller wants states, we must differentiate between None and the empty set.
If the caller didn't want states, states will be None.
However, empty states means the caller still wants states, but the object had no states; e.g. an unchecked check box with no other states.
Previously, we treated an empty set as if the caller didn't want states.