### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

If I create a REST API in API GW with any resources that do not contain any methods, and try to export the JSON schema, it fails due to:
```python
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/services/apigateway/helpers.py", line 1468, in _add_paths
    for method, method_config in item.get("resourceMethods").items():
AttributeError: 'NoneType' object has no attribute 'items'
```

Resource Structure:
/ - no methods defined
/{somethingId} - GET method defined

### Expected Behavior

I should be able to export an API that contains resources that do not contain any methods, just like in AWS.

### How are you starting LocalStack?

With the `localstack` script

### Steps To Reproduce

I start localstack:
`localstack start`

I create some lambda that prints hello world:

lambda.js
```javascript
'use strict'

const apiHandler = (payload, context, callback) => {
    callback(null, {
        statusCode: 200,
        body: JSON.stringify({
            message: 'Hello from Lambda'
        }),
    }); 
}
    
module.exports = {
    apiHandler,
}

```

```bash
zip function.zip lambda.js
awslocal lambda create-function \
  --function-name apigw-lambda \
  --runtime nodejs16.x \
  --handler lambda.apiHandler \
  --memory-size 128 \
  --zip-file fileb://function.zip \
  --role arn:aws:iam::111111111111:role/apigw
```

I create the REST api:
```bash
awslocal apigateway create-rest-api --name 'API Gateway Lambda integration' --tags '{"_custom_id_": "test-api"}'
```

I get the root resource ID:
```bash
awslocal apigateway get-resources --rest-api-id test-api
```

Create a child resource:
```bash
awslocal apigateway create-resource \
  --rest-api-id test-api \
  --parent-id 4hvslaey04 \
  --path-part "{somethingId}"
```

Create method (ONLY FOR THE CHILD NODE):
```bash
awslocal apigateway put-method \
  --rest-api-id test-api \
  --resource-id oxykf16mty \
  --http-method GET \
  --request-parameters "method.request.path.somethingId=true" \
  --authorization-type "NONE"
```

Create Integration (ONLY FOR THE CHILD NODE):
```bash
awslocal apigateway put-integration \
  --rest-api-id test-api \
  --resource-id oxykf16mty \
  --http-method GET \
  --type AWS_PROXY \
  --integration-http-method POST \
  --uri arn:aws:apigateway:us-east-1:lambda:path/2015-03-31/functions/arn:aws:lambda:us-east-1:000000000000:function:apigw-lambda/invocations \
  --passthrough-behavior WHEN_NO_MATCH
```

Create Deployment:
```bash
awslocal apigateway create-deployment \
  --rest-api-id test-api \
  --stage-name test
```

Try to export OpenAPI 3.0 Schema
```bash
awslocal apigateway get-export --parameters extensions='apigateway' --rest-api-id test-api --stage-name test --export-type oas30 oasTest.json --d
```

Gets the following in return:
```python
An error occurred (InternalError) when calling the GetExport operation (reached max retries: 4): exception while calling apigateway.GetExport: Traceback (most recent call last):
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/aws/chain.py", line 90, in handle
    handler(self, self.context, response)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/aws/handlers/service.py", line 123, in __call__
    handler(chain, context, response)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/aws/handlers/service.py", line 93, in __call__
    skeleton_response = self.skeleton.invoke(context)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/aws/skeleton.py", line 154, in invoke
    return self.dispatch_request(context, instance)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/aws/skeleton.py", line 166, in dispatch_request
    result = handler(context, instance) or {}
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/aws/forwarder.py", line 60, in _call
    return handler(context, req)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/aws/skeleton.py", line 118, in __call__
    return self.fn(*args, **kwargs)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/services/apigateway/provider.py", line 1765, in get_export
    result = openapi_exporter.export_api(
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/services/apigateway/helpers.py", line 1462, in export_api
    return self.exporters.get(export_type)(api_id, stage, export_format)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/services/apigateway/helpers.py", line 1515, in _oas30_export
    self._add_paths(spec, resources)
  File "/opt/code/localstack/.venv/lib/python3.10/site-packages/localstack/services/apigateway/helpers.py", line 1468, in _add_paths
    for method, method_config in item.get("resourceMethods").items():
AttributeError: 'NoneType' object has no attribute 'items'
```

### Environment

```markdown
- OS: Both Windows 10 and MacOS Ventura 13.4.1(Intel CPU)
- LocalStack: latest
```


### Anything else?

I've tested this on Windows with "localstack start" script. And I have tested this on Mac with terraform docker_container. Same results.
ClientCertificate,
    ClientCertificates,
    ConflictException,
    ConnectionType,
    CreateAuthorizerRequest,
    CreateRestApiRequest,
    CreateStageRequest,
    DocumentationPart,
    DocumentationPartIds,
    DocumentationPartLocation,
    DocumentationParts,
    DocumentationVersion,
    DocumentationVersions,
    DomainName,
    DomainNames,
    DomainNameStatus,
    EndpointConfiguration,
    ExportResponse,
    GatewayResponse,
    GatewayResponses,
    GatewayResponseType,
    GetDocumentationPartsRequest,
    Integration,
    IntegrationResponse,
    IntegrationType,
    ListOfApiStage,
    ListOfPatchOperation,
    ListOfStageKeys,
    ListOfString,
    MapOfStringToBoolean,