Null data in a parameter as an available value choice

Hi

I have defined the following parameter available values for empty cells from SQL (Specify values);

• Completely blank
• (Null)
• Null
• Nothing

So when my SQL dataset returns values with NULL (empty data) I want to be able to filter these if required. Basically, the parameter has Yes and No, these work fine, its just the null/empty bits I don't know how to filter?

Hope that makes sense