Empty (orphan) IAM groups should be removed as best practice, as it will reduce the complexity of an environment and allow for better monitoring.
In AWS Console -
In Terraform -
References:
https://docs.aws.amazon.com/IAM/latest/UserGuide/id_groups.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/iam_group
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/iam_group_membership