modules/worker_groups: Fix security group index error when var.create_eks is false

[chancez]

Previously this error could occur when var.create_eks was false:

Error: Invalid index

  on ../../../modules/eks-worker-groups/locals.tf line 83, in locals:
  83:   worker_security_group_id = var.worker_create_security_group ? aws_security_group.worker_groups.0.id : var.worker_security_group_id
    |----------------
    | aws_security_group.worker_groups is empty tuple

The given key does not identify an element in this collection value.

[js-timbirkett]

That's a nice pattern for all the var.thing && var.create_eks that goes on in this module.

Thanks for the PR!

[chancez]

@js-timbirkett Yeah, I was contemplating doing this for the other cases that are similar too, but decided to leave that for later as a follow up to avoid making too many changes at once.