Method on_null

bench/bench.cpp:520–520

            bool on_double(double, string_view, system::error_code&) { return true; }
            bool on_bool(bool, system::error_code&) { return true; }
            bool on_null(system::error_code&) { return true; }
            bool on_comment_part(string_view, system::error_code&) { return true; }
            bool on_comment(string_view, system::error_code&) { return true; }
        };