How to validate empty variable using a condition?

I have a SOAP request that returns a XML response from which I extract the value for a specific tag in a variable.
In some cases, the variable is empty.
What is the correct way to evaluate in a condition if the variable is empty. I tried several reg expression (even Does Not Exist, ...) but I never reach the branch of the condition where the variable is empty and an error message needs to be logged.
However during replay I can clearly see that the variable is empty and no errors are shown.