v3: make maps nullable

[sauterp]

Description

❯ diff -C 3 v3/generator/source.old.yaml v3/generator/source.yaml
*** v3/generator/source.old.yaml    2024-04-22 13:26:16.271603306 +0200
--- v3/generator/source.yaml    2024-04-22 14:10:03.906145240 +0200
***************
*** 3479,3484 ****
--- 3479,3485 ----
        description: DBaaS plan
      labels:
        type: object
+       nullable: true
        additionalProperties:
          type: string
      dbaas-mysql-database-name:

Checklist

(For exoscale contributors)

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