fix permadiff by reading empty docker_config field (#10113) (#7044)

[upstream:c4d1efa32fa9aec82c1951a596f115b2ac443d88]

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/10113.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+artifactregistry: fixed docker_config permadiff
+```

google-beta/services/artifactregistry/resource_artifact_registry_repository.go

@@ -1079,9 +1079,6 @@ func flattenArtifactRegistryRepositoryDockerConfig(v interface{}, d *schema.Reso
 		return nil
 	}
 	original := v.(map[string]interface{})
-	if len(original) == 0 {
-		return nil
-	}
 	transformed := make(map[string]interface{})
 	transformed["immutable_tags"] =
 		flattenArtifactRegistryRepositoryDockerConfigImmutableTags(original["immutableTags"], d, config)
@@ -1525,9 +1522,14 @@ func expandArtifactRegistryRepositoryKmsKeyName(v interface{}, d tpgresource.Ter
 
 func expandArtifactRegistryRepositoryDockerConfig(v interface{}, d tpgresource.TerraformResourceData, config *transport_tpg.Config) (interface{}, error) {
 	l := v.([]interface{})
-	if len(l) == 0 || l[0] == nil {
+	if len(l) == 0 {
 		return nil, nil
 	}
+
+	if l[0] == nil {
+		transformed := make(map[string]interface{})
+		return transformed, nil
+	}
 	raw := l[0]
 	original := raw.(map[string]interface{})
 	transformed := make(map[string]interface{})