fix: handle null undrainable_node_behavior in precondition validation

examples/application_gateway_ingress/main.tf

@@ -162,7 +162,7 @@ module "aks" {
   location                  = local.resource_group.location
   kubernetes_version        = "1.33" # don't specify the patch version!
   automatic_channel_upgrade = "patch"
-  agents_availability_zones = ["2"]
+  agents_availability_zones = ["3"]
   agents_count              = null
   agents_max_count          = 2
   agents_max_pods           = 100

examples/local_dns_config/variables.tf

@@ -16,7 +16,7 @@ variable "create_resource_group" {
 }
 
 variable "location" {
-  default     = "australiasoutheast"
+  default     = "westeurope"
   description = "The location where the Managed Kubernetes Cluster should be created."
 }
 

extra_node_pool.tf

@@ -180,7 +180,10 @@ resource "azurerm_kubernetes_cluster_node_pool" "node_pool_create_before_destroy
       error_message = "Exactly one of `max_surge` or `max_unavailable` must be specified in `upgrade_settings` for node pool '${each.value.name}'."
     }
     precondition {
-      condition     = each.value.upgrade_settings == null || try(each.value.upgrade_settings.undrainable_node_behavior, null) == null || contains(["Cordon", "Schedule"], try(each.value.upgrade_settings.undrainable_node_behavior, ""))
+      condition = each.value.upgrade_settings == null ? true : (
+        try(each.value.upgrade_settings.undrainable_node_behavior, null) == null ? true :
+        contains(["Cordon", "Schedule"], each.value.upgrade_settings.undrainable_node_behavior)
+      )
       error_message = "`undrainable_node_behavior` in `upgrade_settings` must be `null`, `\"Cordon\"`, or `\"Schedule\"` for node pool '${each.value.name}'."
     }
   }
@@ -347,7 +350,10 @@ resource "azurerm_kubernetes_cluster_node_pool" "node_pool_create_after_destroy"
       error_message = "Exactly one of `max_surge` or `max_unavailable` must be specified in `upgrade_settings` for node pool '${each.value.name}'."
     }
     precondition {
-      condition     = each.value.upgrade_settings == null || try(each.value.upgrade_settings.undrainable_node_behavior, null) == null || contains(["Cordon", "Schedule"], try(each.value.upgrade_settings.undrainable_node_behavior, ""))
+      condition = each.value.upgrade_settings == null ? true : (
+        try(each.value.upgrade_settings.undrainable_node_behavior, null) == null ? true :
+        contains(["Cordon", "Schedule"], each.value.upgrade_settings.undrainable_node_behavior)
+      )
       error_message = "`undrainable_node_behavior` in `upgrade_settings` must be `null`, `\"Cordon\"`, or `\"Schedule\"` for node pool '${each.value.name}'."
     }
   }

main.tf

@@ -745,7 +745,7 @@ data "azurerm_subnet" "blob_driver_check" {
 
 check "blob_driver_subnet_service_endpoint" {
   assert {
-    condition     = var.vnet_subnet == null || !var.storage_profile_enabled || !var.storage_profile_blob_driver_enabled || contains(try(data.azurerm_subnet.blob_driver_check[0].service_endpoints, []), "Microsoft.Storage")
+    condition     = var.vnet_subnet == null || !var.storage_profile_enabled || !var.storage_profile_blob_driver_enabled || contains(coalesce(try(data.azurerm_subnet.blob_driver_check[0].service_endpoints, null), []), "Microsoft.Storage")
     error_message = "The subnet used by the default node pool does not have 'Microsoft.Storage' in its service_endpoints. When storage_profile_blob_driver_enabled is true, the AKS Blob CSI driver may automatically add this service endpoint out-of-band, causing Terraform state drift. To prevent this, add service_endpoints = [\"Microsoft.Storage\"] to your subnet configuration. See: https://github.com/Azure/terraform-azurerm-aks/issues/424"
   }
 }