Showing null values as "Blank"

Summary

How do I show null values with the word "Blank" or "NA" or something similar?

Content

Hello,

In the Product Dimension for 'Product Name', we have some blanks due to poor process. We are trying to improve the process of always adding a product to an opportunity. To help us do that, we are using Analytics. I created a pivot table showing all of the opportunities we have open by Product Name. We have some that don't have a product so I want to show the word "Blank" or "NA". I'm using this case statement and it's not working. Any thoughts?