Method on_null

test/test.hpp:173–173

        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; }
    };