Fix "Invalid index" error when VPC baseline is disabled

[wheerd]

Currently, we have the VPC baseline disabled in one of our AWS accounts. When trying to update to the latest version of the module, we get several errors like this:

Error: Invalid index

  on .terraform/modules/master_secure_baseline/vpc_baselines.tf line 395, in module "vpc_baseline_us-west-2":
 395:   flow_logs_iam_role_arn      = local.is_cw_logs ? aws_iam_role.flow_logs_publisher[0].arn : null
    ├────────────────
    aws_iam_role.flow_logs_publisher is empty tuple

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

Th condition for those modules is not using var.vpc_enable properly. This PR aims to fix that and simplify the conditions as well.

[nozaq]

@wheerd Thank you, LGTM!