fix: Do not recreate cluster when no SG given

[dpiddockcmp]

PR o'clock

Description

Setting cluster_create_security_group = false and not setting cluster_security_group_id causes the cluster to be recreated on every apply.

I did try setting cluster_security_group_id = null but then terraform complains that there's a null item inside a list and that I'm a very bad person for doing something so rude.

Fixes #797

Checklist

• Add sementics prefix to your PR or Commits (at leats one of your commit groups)
• CI tests are passing
• README.md has been updated after any changes to variables and outputs. See https://github.com/terraform-aws-modules/terraform-aws-eks/#doc-generation

[github-actions]

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