fix Invalid index error when creating a private cluster

As google_container_cluster.primary.private_cluster_config may be empty,
we check its length before trying to access it.

When creating a new private cluster, a "Invalid index" error was showing
up during the planning.

By checking for variable length before trying to access it the plan (and
apply) works as expected.

Issue: [terraform-google-modules#419]

Co-authored-by: Morgante Pell <morgantep@google.com>

autogen/main/main.tf.tmpl

@@ -106,7 +106,7 @@ locals {
   cluster_output_zones          = local.cluster_output_regional_zones
 
 {% if private_cluster %}
-  cluster_endpoint = var.enable_private_nodes ? (var.deploy_using_private_endpoint ? google_container_cluster.primary.private_cluster_config.0.private_endpoint : google_container_cluster.primary.private_cluster_config.0.public_endpoint) : google_container_cluster.primary.endpoint
+  cluster_endpoint = (var.enable_private_nodes && length(google_container_cluster.primary.private_cluster_config) > 0) ? (var.deploy_using_private_endpoint ? google_container_cluster.primary.private_cluster_config.0.private_endpoint : google_container_cluster.primary.private_cluster_config.0.public_endpoint) : google_container_cluster.primary.endpoint
 {% else %}
   cluster_endpoint = google_container_cluster.primary.endpoint
 {% endif %}