fix: Do not attach policy if Karpenter node role is not created by module

[dex4er]

Description

Karpenter module fails

│ Error: Invalid index
│ 
│   on .terraform/modules/eks.eks_karpenter/modules/karpenter/main.tf line 576, in resource "aws_iam_role_policy_attachment" "node":
│  576:   role       = aws_iam_role.node[0].name
│     ├────────────────
│     │ aws_iam_role.node is empty tuple
│ 
│ The given key does not identify an element in this collection value: the
│ collection has no elements.
╵

if node role is not created:

module "eks_karpenter" {
  source  = "terraform-aws-modules/eks/aws//modules/karpenter"
  version = "~> 20.0"

  cluster_name = module.eks.cluster_name

  create_iam_role          = true
  create_node_iam_role          = false
}

Motivation and Context

Bugfix

Breaking Changes

No, it's a minor fix

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects
• I have executed pre-commit run -a on my pull request

[bryantbiggs]

thank you!

[antonbabenko]

This PR is included in version 20.8.1 🎉

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.