sks cluster update: fix oidc config drop

[kobajagi]

Description

Fix the OIDC config getting nulled when any other attribute is updated.
API handles explicit null as empty, but it does not set nullable in OpenAPI spec which makes CLI/egoscale send explicit null.
This is why we have to set existing value in every update call.

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block, and add the Pull Request #number for each bit you add to the CHANGELOG.md)
• Testing

Testing

$ EXOSCALE_TRACE=1 exo c sks get mycluster
..."oidc":{"client-id":"12345","username-claim":"something",...
$ go run . c sks update mycluster --description test
 ✔ Updating SKS cluster "mycluster"... 33s
...
$ EXOSCALE_TRACE=1 exo c sks get mycluster
..."oidc":{"client-id":"12345","username-claim":"something",...