Method on_null

test/basic_parser.cpp:1700–1700

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