next_step_primary_url breaks terraform when passing value for `existing_kms_instance_crn` in DA

[ocofaigh]

Looks like there is an issue with the code that was added in #790

2025/12/03 17:00:01 Terraform plan | Error: Invalid function argument
 2025/12/03 17:00:01 Terraform plan | 
 2025/12/03 17:00:01 Terraform plan |   on outputs.tf line 82, in output "next_step_primary_url":
 2025/12/03 17:00:01 Terraform plan |   82:   value       = "[https://cloud.ibm.com/keyprotect/${urlencode(module.kms.key_protect_crn)}?paneId=manage](https://cloud.ibm.com/keyprotect/$%7Burlencode(module.kms.key_protect_crn)%7D?paneId=manage)"
 2025/12/03 17:00:01 Terraform plan |     ├────────────────
 2025/12/03 17:00:01 Terraform plan |     │ while calling urlencode(str)
 2025/12/03 17:00:01 Terraform plan |     │ module.kms.key_protect_crn is null
 2025/12/03 17:00:01 Terraform plan | 
 2025/12/03 17:00:01 Terraform plan | Invalid value for "str" parameter: argument must not be null.

This error was seen when passing a value for existing_kms_instance_crn into the DA. The above output needs to consider that module.kms.key_protect_crn might be null in this case, and hence needs to be updated to use a local variable thats determined like below:

locals {
  kms_crn = var.existing_kms_instance_crn == null ? module.kms.key_protect_crn : var.existing_kms_instance_crn
}

The output named kms_instance_crn already uses above logic, so if we are creating a new local, suggest to update the output to also use the new local.

[mounicatellabati]

#808