The reason you cannot pass null is because a ref parameter is given special treatment by the C# compiler. Any ref parameter must be a reference that can be passed to the function you are calling. Since you want to pass null the compiler is refusing to allow this since you are not providing a reference that the function is expecting to have.
Your only real option would be to create a local variable, set it to null, and pass that in. The compiler will not allow you to do much more than that.