`aws_rds_cluster_role_association` should allow empty `feature_name` argument

[matthias-bruhse]

Description

It is not possible to create a role association with and empty feature_name argument because the map key is always used as fallback:

terraform-aws-rds-aurora/main.tf

Line 261 in 2c3946c

feature_name = try(coalesce(each.value.feature_name, each.key))

While feature_name is a required input of aws_rds_cluster_role_association, setting it to an empty string or null is valid, because this parameter is only used for Aurora PostgreSQL. Example from AWS docs:

aws rds add-role-to-db-cluster \
   --db-cluster-identifier my-db-cluster \
   --feature-name s3Import \
   --role-arn your-role-arn   \
   --region your-region

For Aurora MySQL, role associations can have a feature_name, but there are also cases where they should not have any. Example from AWS docs:

aws rds add-role-to-db-cluster --db-cluster-identifier my-cluster --role-arn arn:aws:iam::123456789012:role/AllowAuroraS3Role
aws rds add-role-to-db-cluster --db-cluster-identifier my-cluster --role-arn arn:aws:iam::123456789012:role/AllowAuroraLambdaRole

The use of coalesce makes it impossible to not set a feature_name.

• [x ] ✋ I have searched the open/closed issues and my issue is not listed.

Versions

• Module version: v10.2.0
• Terraform version: OpenTofu v1.11.2
• Provider version(s): aws v6.28.0

Reproduction Code [Required]

module "aurora" {
  [...]

  role_associations = {
    my_role_without_feature_name = {
      role_arn           = "role-arn arn:aws:iam::123456789012:role/AllowAuroraLambdaRole"
      feature_name = ""
    }
  }

Expected behavior

Role association is created with feature_name = "".

Actual behavior

Apply fails because role association cannot be created. Reason is that my_role_without_feature_name is used as feature_name, which is not a valid input.

Additional context

I personally would prefer, if the default for feature_name is null or "", but being able to at least explicitly overwrite the default with null or "" is a requirement.