(aws-amplify-alpha): Custom domain with blank prefix causes stack drift

### Describe the bug

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.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

The underlying CloudFormation stack should not be drifted.

### Current Behavior

The stack is in drifted state after deployment:

Expected:

```json
{
"SubDomainSettings": [
{
"BranchName": "master",
"Prefix": ""
}
],
"AppId": "xxxyyyzzz",
"AutoSubDomainIAMRole": "arn:aws:iam::xxxyyyzzz",
"DomainName": "mysubdomain.mydomain.com",
"EnableAutoSubDomain": false,
"AutoSubDomainCreationPatterns": [
"*",
"pr*"
]
}
```

Actual:

```json
{
"SubDomainSettings": [
{
"BranchName": "master"
}
],
"AppId": "xxxyyyzzz",
"AutoSubDomainIAMRole": "arn:aws:iam::xxxyyyzzz",
"DomainName": "mysubdomain.mydomain.com",
"EnableAutoSubDomain": false,
"AutoSubDomainCreationPatterns": [
"*",
"pr*"
]
}
```

### Reproduction Steps

Add a custom domain with a blank prefix like so:

```python
amplify_app.add_domain('AppDomain',
domain_name = 'mysubdomain.mydomain.com',
sub_domains = [amplify.SubDomain(prefix = '', branch = 'master')]
)
```

As specified by the documentation for `amplify.SubDomain`, we: `Use '' to map to the root of the domain`.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

2.222.0-alpha.0

### AWS CDK CLI version

2.1031.1

### Node.js Version

v24.11.0

### OS

macOS 15.7.1

### Language

Python

### Language Version

3.14.0

### Other information

_No response_

Start from the Amplify app add_domain API and the SubDomain input shown in the reproduction, then reproduce the deployment with an empty prefix and inspect the generated CloudFormation properties. Confirm the fix by verifying that the deployed stack preserves the blank Prefix and no longer reports drift.

Written by the indexing model from the issue text.