How to assign value of measure to 0 if no selection in prompt [Relational Model]

[clingst]

Please advice for the trick to make this work..

I have a mutiselect prompt which is set optional.
Due to some business context,
I need to assign the measure to 0 if no selection in the prompt.

however.. due to the nature of the prompt and rules:
NO selection is equals to SELECT ALL.
Thus, a total of ALL is returned even though no selection in the prompt.

Appreciate to share your trick how can I handle this... :D
thank u..

as for now.. i m work around with value comparison and UNION ..
but i dont think this is a good method..