Fix PackRawBody incorrectly accepts null value.
Fix PackRawBody incorrectly accepts null value.

msgpack/msgpack-cli,undeadlabs/msgpack-cli,msgpack/msgpack-cli,scopely/msgpack-cli,modulexcite/msgpack-cli,undeadlabs/msgpack-cli,scopely/msgpack-cli,modulexcite/msgpack-cli