Bug: DeploymentOutputEvaluationFailed

### Deployment Mode

Deploy To Azure

### Steps to reproduce


Error when choosing deploy in Azure Landing Zone (former Enterprise-Scale Landing Zone) environment when specifying Third Party Appliance firewall and the DNS Forwarder IP addresses of existing environment.

### Error Message

```json
{
"status": "Failed",
"error": {
"code": "DeploymentOutputEvaluationFailed",
"message": "Unable to evaluate template outputs: 'serviceSubnet'. Please see error details and deployment operations. Please see https://aka.ms/arm-debug for usage details.",
"details": [
{
"code": "DeploymentOutputEvaluationFailed",
"target": "serviceSubnet",
"message": "The template output 'serviceSubnet' is not valid: The language expression property array index '1' is out of bounds.."
}
]
}
}
```

### Screenshots

_No response_

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/Azure/data-management-zone/blob/main/CODE_OF_CONDUCT.md)