Search for RTC MultiSelect attributes with no value

We're using an RTC Multi Select Attribute that can either read from a defined list, or allow the user to Create new values as needed; the user will mainly create new values, and the defined list only has "Unassigned" in it.

In our implementation, it is not possible to set the default value to Unassigned, so on WI creation, the attribute will be blank.  Is there a way to query for all WI's that have this attribute value "blank"? There is not an option in the query editor for 'not exists' like it has for other Attribute types, and as the values are not defined, 'not containing' is not of use either.