artifactregistry: fix create/update of virtual repo with empty upstream_policies

[bogaertg]

Fixes create/update of google_artifact_registry_repository in VIRTUAL_REPOSITORY mode when virtual_repository_config.upstream_policies is an empty list.

Bug

The Artifact Registry REST API accepts and creates a virtual repository with virtualRepositoryConfig: {upstreamPolicies: []} fine (verified directly against the API, not through Terraform). The generated expand for virtual_repository_config drops the upstreamPolicies key from the request whenever the list is empty, which in turn makes the whole virtualRepositoryConfig map empty - so the surrounding create/update logic (gated on the same emptiness check) omits the virtualRepositoryConfig field from the request entirely, and the API rejects it with:

Error 400: Virtual repository config is not specified

This happens on both create (a virtual repo with no upstream configured yet) and update (removing the last upstream policy).

Fix

Added a custom_expand for virtual_repository_config that includes upstreamPolicies whenever the block was actually configured, regardless of whether the resulting list is empty - matching what the REST API accepts.

Tests

Added TestAccArtifactRegistryRepository_virtualEmpty: create with zero upstream_policies, import verify, update to one policy, import verify, back down to zero.

artifactregistry: fixed `google_artifact_registry_repository` failing to create or update a `VIRTUAL_REPOSITORY`-mode repository with an empty `virtual_repository_config.upstream_policies` list

[github-actions]

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