fix(workstation-cluster): allow private_cluster_config to be null (#3821)

Fixes #3819 by changing the default of `private_cluster_config` to `null` instead of an empty object, preventing the module from unintentionally generating an empty `private_cluster_config` block and treating the cluster as private when it wasn't requested.

tests/modules/workstation_cluster/context.yaml

@@ -41,8 +41,7 @@ values:
     labels: null
     location: europe-west8
     network: projects/foo-dev-net-spoke-0/global/networks/dev-spoke-0
-    private_cluster_config:
-    - enable_private_endpoint: false
+    private_cluster_config: []
     project: dev-test-0
     subnetwork: projects/foo-dev-net-spoke-0/regions/europe-west8/subnetworks/default
     tags: null

tests/modules/workstation_cluster/examples/custom-image.yaml

@@ -36,8 +36,7 @@ values:
     labels: null
     location: europe-west8
     network: https://www.googleapis.com/compute/v1/projects/xxx/global/networks/aaa
-    private_cluster_config:
-    - enable_private_endpoint: false
+    private_cluster_config: []
     project: project-id
     subnetwork: subnet_self_link
     timeouts: null

tests/modules/workstation_cluster/examples/iam.yaml

@@ -37,8 +37,7 @@ values:
     labels: null
     location: europe-west8
     network: https://www.googleapis.com/compute/v1/projects/xxx/global/networks/aaa
-    private_cluster_config:
-    - enable_private_endpoint: false
+    private_cluster_config: []
     project: project-id
     subnetwork: subnet_self_link
     timeouts: null

tests/modules/workstation_cluster/examples/simple.yaml

@@ -36,8 +36,7 @@ values:
     labels: null
     location: europe-west8
     network: https://www.googleapis.com/compute/v1/projects/xxx/global/networks/aaa
-    private_cluster_config:
-    - enable_private_endpoint: false
+    private_cluster_config: []
     project: project-id
     subnetwork: subnet_self_link
     timeouts: null