Fix cluster creation failure when login nodes are used and cloudwatch is disabled

[hgreebe]

Description of changes

• Fix cluster creation error when login nodes are used and cloudwatch is disabled
• Before this fix the following error occured:

pcluster create-cluster --cluster-name test --cluster-configuration cluster-config.yaml
{
  "message": "'NoneType' object has no attribute 'log_group_name'"
}

• This adds the same guard already used for queues: https://github.com/aws/aws-parallelcluster/blob/develop/cli/src/pcluster/templates/queues_stack.py#L264-L266
• Unit test verifies that template can be rendered is cloudwatch is disabled but login nodes are used.

Tests

• Test cluster creation passed
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