Fix userdata msgpack emission using empty string instead of emitter output

[traitimtrongvag]

The msgpack emitter calls the userdata emitter callback but ignores the returned value and always serializes "obj->value.sv" and "obj->len" instead.

For userdata objects these fields are never populated, so msgpack output becomes an empty string regardless of what the emitter returns.

This change makes the msgpack path use the emitter output, matching the existing behavior used by the JSON emitter.

Testing

• Verified userdata objects with a custom emitter now serialize the expected value.
• Verified emitter callbacks returning NULL are handled correctly.
• Verified userdata objects without an emitter continue to behave correctly.