It may help if you can share your app or some more info to your requirements. Are you joining columns A & B in your example from different tables? If so you should still be able to test for blanks in your column B in the initial load. Maybe I'm not understanding your problem correctly.
In a table box that isn't possible, but in a straight table it is. You need a way to write the expression and table box doesn't include that. In a straight table use A as dimension and then your expression as expression. On Presentation tab uncheck Suppress Zero-Values.