fix: Local cluster_name error when var.cluster_arn is empty

[rvaralda]

Description

When configuring modules/service to disable service creation with parameters:

  create         = false
  create_service = false

The following error happens

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Invalid function argument
│ 
│   on .terraform/modules/service/main.tf line 1206, in locals:
│ 1206:   cluster_name = element(split("/", var.cluster_arn), 1)
│     ├────────────────
│     │ while calling split(separator, str)
│     │ var.cluster_arn is null
│ 
│ Invalid value for "str" parameter: argument must not be null.
╵

This change make it works in with create true/false without error.

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

[antonbabenko]

This PR is included in version 5.11.4 🎉

[github-actions]

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