defaulted to false and conditional output

modules/gke-cluster/main.tf

@@ -279,7 +279,7 @@ resource "google_container_cluster" "cluster" {
   }
 
   dynamic "notification_config" {
-    for_each = var.notification_config != null ? [""] : []
+    for_each = var.notification_config ? [""] : []
     content {
       pubsub {
         enabled = var.notification_config

modules/gke-cluster/outputs.tf

@@ -48,5 +48,5 @@ output "name" {
 
 output "notifications" {
   description = "GKE PubSub notifications topic."
-  value       = google_pubsub_topic.notifications.id
+  value       = var.notification_config ? google_pubsub_topic.notifications[0].id : null
 }