How to Return Null Value

In the following code, returning the None value causes the compiler to throw an error regarding an Option being used while it expected an &str. I have changed it to return an empty string via a match-block, but I require the target variable to be completely omitted if the string is empty rather than containing "-h". How can a null value of sorts be used in this situation? // match-block let cert_type = match host_flag { Some(_) => "-h", None => "", }; // Target variable to ignore if v...