fix(kms): add nil pointer check on automatic field

[luxium30]

Description

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

go run main.go kms key show 019f7e76-a085-71c6-a010-18cd5e7a0a7c
┼─────────────────┼────────────────────────────────────────────────────┼
│     KMS KEY     │                                                    │
┼─────────────────┼────────────────────────────────────────────────────┼
│ ID              │ 019f7e76-a085-71c6-a010-18cd5e7a0a7c               │
│ Name            │ test-4                                             │
│ Created At      │ 2026-07-20 07:39:36.268625235 +0000 UTC            │
│ Multizone       │ true                                               │
│ Origin Zone     │ ch-gva-2                                           │
│ Status          │ enabled                                            │
│ Replicas Status │ de-fra-1                                           │
│ Material        │ createdAt: 2026-07-20 07:39:36.268625235 +0000 UTC │
│                 │ version: 2                                         │
│ Rotation        │ auto: true                                         │
│                 │ count: 1                                           │
│                 │ nextAt: 2027-07-20 07:38:51.399877708 +0000 UTC    │
│                 │ rotationPeriod: 365                                │
│ Usage           │ encrypt-decrypt                                    │
│ Source          │ exoscale-kms                                       │
│ Description     │                                                    │
┼─────────────────┼────────────────────────────────────────────────────┼

[luxium30]

https://app.shortcut.com/exoscale/story/189066/kms-store-manual-field-in-key-material-persistently

[natalie-o-perret]

👍