fix: fix bug with CRN parse logic

[ocofaigh]

Description

When enable_activity_tracker_event_routing_to_cos_bucket was set to false, the following error was occurring:

 2025/10/17 16:52:19 Terraform plan | Error: Invalid function argument
 2025/10/17 16:52:19 Terraform plan | 
 2025/10/17 16:52:19 Terraform plan |   on .terraform/modules/existing_kms_key_crn_parser/modules/crn-parser/main.tf line 6, in locals:
 2025/10/17 16:52:19 Terraform plan |    6:   split_crn        = coalescelist(split(":", var.crn))
 2025/10/17 16:52:19 Terraform plan |     ├────────────────
 2025/10/17 16:52:19 Terraform plan |     │ while calling split(separator, str)
 2025/10/17 16:52:19 Terraform plan |     │ var.crn is null

This PR has a fix and plugs the test gap

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[ocofaigh]

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🎉 This PR is included in version 1.4.3 🎉

The release is available on:

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• v1.4.3

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