fix(amplify): custom domain with blank prefix causes stack drift

[syukawa-gh]

When a subdomain is mapped with an empty string prefix, the CloudFormation template includes Prefix: "" but AWS omits empty prefix values, causing immediate stack drift after deployment.

Now omit the prefix property entirely when it is an empty string.

Closes #35958

[aws-cdk-automation]

The pull request linter fails with the following errors:

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

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

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[syukawa-gh]

Exemption Request: This PR fixes a stack drift issue by omitting empty prefix from CloudFormation template. Unit test has been added. Integration test would require an actual Amplify app with a custom domain, which is not practical for automated testing.

[syukawa-gh]

Closing this PR. After further investigation, the prefix property is required in the CFn SubDomainSettingProperty schema, so omitting it causes a synthesis validation error. The stack drift issue (#35958) is caused by CloudFormation's behavior of omitting empty string values in drift detection, which cannot be fixed on the CDK side.

[github-actions]

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