IoTCoreCustomAuthorizerResponse create invalid payalod

It seems that IoTCoreCustomAuthorizerResponse can create a payload not accepted by AWS IOT broker.

The following attributes: `pub policy_documents: Vec>` allow a Vec that contains None but AWS IOT broker raise an AUTHORIZATION_FAILURE error in that case.

The corresponding json value is:
```json
"policyDocuments": [
null
]
```

Should policy_documents type be: `Vec>` instead ?

Start by locating IoTCoreCustomAuthorizerResponse and its policy_documents field, then inspect how IamPolicyDocument values are serialized. Look for related serialization or response tests and verify the completed change cannot produce a policyDocuments array containing null, matching the AWS IoT broker behavior described here.

Written by the indexing model from the issue text.