I have to tables lets say Transactions(Contractor and Other Fields) and Contractors (with Contractor and Region field) which when associated produces null values as there is no association on the key fields(i.e. Contractor does not appear on the Contractor Table but does on the transactional table).
When I create a chart object say straight table i want to be able to click on the Regions that are NULL and see the list on Contractors so we can go fix the data on the source.
Besides using statements like NullAsValue or a isNull check on field values in your script (please refer to the script), which probably doesn't help here, I would suggest that you use
as expression, then make the expression searchable and use an empty search string. This should filter all empty Region (I assume you don't have empty, but not NULL, original Region values, or you might want to check them as well).