Webi - make a Sum like

Hello,

I would like to do a sum of variable, but just under some condition. I want to do something like:

=Sum([Variable1] Where([Variable2="[urVariable2]" and [Variable3="[urVariable3]"))

[urVariable2]=UserResponse("[Variable2")

[urVariable3]=UserResponse("[Variable3")

That means that  urVariable2 and urVariable3 are what the user wrote in a prompt. My problem is that this is optional, that means, if variable2 is empty, I want all the values of variable2 without importing the value, and just take variable3 in the condition.

How can I do this? My problem is: if I keep empty Variable2 empty this is looking for Variable2="NULL" so no results in the output.

Thank you.