AT DA bug when `existing_cos_instance_crn` is null

[ocofaigh]

It has been reported that when using the AT DA and setting enable_activity_tracker_event_routing_to_cos_bucket to false, and setting existing_cos_instance_crn to null, the DA fails with:

 2025/11/04 10:02:19 Terraform plan | Error: Invalid function argument
 2025/11/04 10:02:19 Terraform plan | 
 2025/11/04 10:02:19 Terraform plan |   on main.tf line 17, in locals:
 2025/11/04 10:02:19 Terraform plan |   17:   cos_instance_guid = element(split(":", var.existing_cos_instance_crn), length(split(":", var.existing_cos_instance_crn)) - 3)
 2025/11/04 10:02:19 Terraform plan |     ├────────────────
 2025/11/04 10:02:19 Terraform plan |     │ while calling split(separator, str)
 2025/11/04 10:02:19 Terraform plan |     │ var.existing_cos_instance_crn is null
 2025/11/04 10:02:19 Terraform plan | 
 2025/11/04 10:02:19 Terraform plan | Invalid value for "str" parameter: argument must not be null.

Its due to the logic here ->

terraform-ibm-activity-tracker/solutions/fully-configurable/main.tf

Line 17 in 6897e51

cos_instance_guid = element(split(":", var.existing_cos_instance_crn), length(split(":", var.existing_cos_instance_crn)) - 3)

always running, even when existing_cos_instance_crn is null which is not correct.

We also should be using our crn-parser module for this.