(aws_apigateway): empty policy attribute for CfnRestApi fails synthesize

### Describe the bug

I'm playing with `cdk migrate` and came across an IMO general CDK issue.
The following CloudFormation Resource deploys just fine:

```
"ApiGatewayRestApi": {
"Type": "AWS::ApiGateway::RestApi",
"Properties": {
"Name": "serverless-project-dev",
"EndpointConfiguration": {
"Types": [
"EDGE"
]
},
"Policy": "",
"MinimumCompressionSize": 1024
}
},
```
Note the empty string for the `Policy` attribute

Migrated into CDK with `cdk migrate` this results in the following L1 construct:
```
const apiGatewayRestApi = new apigateway.CfnRestApi(this, 'ApiGatewayRestApi', {
name: 'serverless-project-dev',
endpointConfiguration: {
types: [
'EDGE',
],
},
policy: '',
minimumCompressionSize: 1024,
});

```

which fails synthesis with the following error:
> CfnSynthesisError: Resolution error: Supplied properties not correct for "CfnRestApiProps"
policy: "" should be an 'object'.

When I remove the `policy` key completely synthesis succeeds

### Expected Behavior

L1 constructs should behave like CFN and therefore an empty string as policy value should succeed

### Current Behavior

cdk synth fails

### Reproduction Steps

try to deploy a CfnRestApi with empty string as value for `policy` attribute

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.128.0 (build d995261)

### Framework Version

_No response_

### Node.js Version

v20.9.0

### OS

macOS

### Language

TypeScript

### Language Version

TypeScript (~5.3.3)

### Other information

_No response_

The issue identifies the TypeScript CfnRestApi reproduction and its policy attribute; start by running the reported cdk synth case and tracing validation of CfnRestApi.policy. Done means an empty-string policy synthesizes successfully while omitting policy continues to work, with the behavior covered by an appropriate test.

Written by the indexing model from the issue text.