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 ?

首先定位 IoTCoreCustomAuthorizerResponse 及其 policy_documents 字段，然后检查 IamPolicyDocument 值的序列化方式。查找相关的序列化或响应测试，并验证完成的更改不会生成包含 null 的 policyDocuments 数组，以符合此处描述的 AWS IoT broker 行为。

由索引模型根据 Issue 内容生成。