Prevent NULL Values in Numeric Fields?

I have created a matrix that dynamically calculates column totals pulled from numeric data entry fields. It works fine until I delete a value and move off of the field. The total produces an error, and then the calculation breaks, even if I go back and add a value into the field.

I know that you can test for NULL in a text field by leaving the value in the condition blank, but how can you test for a NULL in a numeric field? The condition screen will not let me leave a number value empty. It defaults to 0.00.

I need to be able to allow positive or negative numbers in the field, so setting the value to >=0 will not work. I tried telling it to only calculate if the number is >=0 or <=0. That actually works to prevent the calculation from crashing, but the field is still NULL, and if I change the value, I get unpredictable results.

Any other way to go about it? All thoughts are appreciated.  Thanks!