Option[T] work as you expect.
None corresponds to JSON's null.

Note that you need a JsonRW[T] given instance.

        Option.empty[String].toJson
        // null

        Option("str").toJson
        // "str"
        

        """ null """.parseJson[Option[String]]
        // None

        """ "str" """.parseJson[Option[String]]
        // Some(str)