Nulls compute_config.node_role_arn when node_pools is empty

Fixes the apply time error:

```
│ Error: updating EKS Cluster (ex-eks-auto-mode) compute config: operation error EKS: UpdateClusterConfig, https response error StatusCode: 400, RequestID: 93002327-da4d-47e2-8389-518b03e8aa60, InvalidParameterException: When Compute Config nodeRoleArn is not null or empty, nodePool value(s) must be provided.
```

Author: lorengordon

main.tf

@@ -63,7 +63,7 @@ resource "aws_eks_cluster" "this" {
     content {
       enabled       = compute_config.value.enabled
       node_pools    = compute_config.value.enabled ? compute_config.value.node_pools : []
-      node_role_arn = compute_config.value.node_pools != null ? try(aws_iam_role.eks_auto[0].arn, compute_config.value.node_role_arn) : null
+      node_role_arn = compute_config.value.enabled ? (length(compute_config.value.node_pools) > 0 ? try(aws_iam_role.eks_auto[0].arn, compute_config.value.node_role_arn) : null) : null
     }
   }
 

variables.tf

@@ -66,7 +66,7 @@ variable "compute_config" {
   description = "Configuration block for the cluster compute configuration"
   type = object({
     enabled       = optional(bool, false)
-    node_pools    = optional(list(string))
+    node_pools    = optional(list(string), [])
     node_role_arn = optional(string)
   })
   default = null