RFE: Make JsonNullable serialize properly to XML

[bkabrda]

As discussed in OpenAPITools/openapi-generator#3474 it would be awesome if JsonNullable would serialize correctly to XML as well. IIUC most of the heavy machinery is already present in this package and it'd only be necessary to implement the custom XML serializer which would look much like the custom JSON serializer.

[cbornet]

Of course ! Would you do a PR ? It should be close to how it's done for Optional

[bkabrda]

Yeah, I'll try to give it a shot and reach out if I can't make something work.

[bkabrda]

Heh, I was actually doing it wrong, JsonNullable works fine even with XML when properly registered on the XmlMapper. Do you want me to make a note in the README.md about this?

[cbornet]

Good news ! Yes we could have a note on how to register the module in the readme for both ObjectMapper and XmlMapper.

[cbornet]

Closed by #11