v3: Libopenapi bump / nullable reference fix

[pierre-emmanuelJ]

Description

Update libopenbapi and fix the nullable reference generated code

Checklist

(For exoscale contributors)

• [x] Changelog updated (under Unreleased block)
• [x] For a new resource or new attributes: test added/updated

[pierre-emmanuelJ]

Here is an issue related to my change on nullable reference https://github.com/pb33f/libopenapi/issues/283