Add an option to treat optional parameters with variables as inputs as if they had been left blank

I'd like to be able to have an optional parameter filled with a variable name and still be able to use it as if it had not been filled at all. For example, the "Default" field of the "Variable Query" action. When left blank, the dialog has nothing filled in, but if I put a variable there, regardless of what the variable contains, the dialog will always have something filled in. The best I can do is have the variable set to just a space, but then it's highlighted and if the user taps the edit field before typing the space will remain.