v2: fix issue with iam role labels

[kobajagi]

Description

Fixes the bug with iam role labels where API returns no labels field (equivalent to nil) but egoscale translates it to empty slice.

Checklist

(For exoscale contributors)

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

[shortcut-integration[bot]]

This pull request has been linked to Shortcut Story #85146: tf-provider: IAMv3 bugs from terraform-conf-platform migration.