Is this the message you see?
If so, the 2 options I can think of, 1) turn off policy validation or 2) define the variable ahead of it's use in the return template response. Meaning, use a set context variable assertion to set 'url' to nothing.
It does not appear that the validation takes into account a variable declaration in the assertion, so even setting it explicitly in the JS assertion will not suppress the message.
var url = "";
or
var url = null;