Exclude all when no value is selected

[mria.ina]

Hi

I am facing the following request:
Prompt Page needs two boxes: Prompt A points to values from table A and Prompt B points to values from table B. The desired outcome is:

If values are entered for prompt A, apply filter. If not, exclude all results from table A
UNION
If values are entered for prompt B, apply filter. If not, exclude all results from table B

I have tried several appraches but nothing seems to work, can you please provide some help on the matter?

Thanks