Read back empty maven_config field in artifactregistry_repository

[modular-magician]

Fixes #terraform-provider-google/14900

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• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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Release Note Template for Downstream PRs (will be copied)

artifactregistry: fixed reading back maven_config state in `google_artifact_registry_repository`

Derived from GoogleCloudPlatform/magic-modules#8383

[github-actions]

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