only provision notification channels if provided (#375)

Author: shankiyani

terraform/notification.tf

@@ -1,5 +1,8 @@
 resource "google_monitoring_notification_channel" "non_paging" {
-  for_each = var.alert_notification_channel_non_paging
+  for_each = {
+    for k, v in var.alert_notification_channel_non_paging : k => v
+    if k == "email" && try(v.labels.email_address, "") != ""
+  }
 
   project = var.project_id