fix(aws-amplify-alpha) Omit Prefix from SubDomainSettings when empty string is provided

[ajkerr]

Issue # (if applicable)

Closes #35958

Reason for this change

After deploying a CDK stack containing an Amplify app with a custom domain that has a blank prefix, the underlying CloudFormation stack is immediately in "drifted" state.

Description of changes

Per the suggestions in the issue, I updated the renderSubDomainSettings method to omit the Prefix method from the resulting map when the supplied prefix is set to ''.

The existing fallback logic is preserved for falling back to branchName if prefix is null or undefined.

Describe any new or updated permissions being added

None.

Description of how you validated changes

Updated the corresponding test for mapping a branch to the domain root.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[ajkerr]

FYI @pahud

[aws-cdk-automation]

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❌ Fixes must contain a change to an integration test file and the resulting snapshot.
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[ajkerr]

This approach might not work: https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/aws-properties-amplify-domain-subdomainsetting.html#cfn-amplify-domain-subdomainsetting-prefix

The Prefix property is actually required at the CloudFormation level. Perhaps it's a bug in the drift detection or CloudFormation itself that is causing the Prefix attribute to disappear?