fix: auto_provisioning_defaults is not really optional

Due to the disk_type validation for auto provision node pool,
this module always forced to create a GKE standard cluster
with a auto provisioned node pool. This is not desirable if
you manage pools separately like using the `gke-nodepool`.

modules/gke-cluster-standard/variables.tf

@@ -94,7 +94,7 @@ variable "cluster_autoscaling" {
   }
   validation {
     condition = (
-      var.cluster_autoscaling == null ? true : contains(
+      var.cluster_autoscaling == null || var.cluster_autoscaling.auto_provisioning_defaults == null ? true : contains(
         ["pd-standard", "pd-ssd", "pd-balanced"],
       var.cluster_autoscaling.auto_provisioning_defaults.disk_type)
     )