protoc-gen-openapi: POST method with empty body generates a required `requestBody`

Hi there. When I'm writing a POST method with an empty request body, protoc-gen-openapi generates a required `requestBody`

```protobuf
rpc Logout(google.protobuf.Empty) returns (google.protobuf.Empty) {
option (google.api.http) = {
post: "/api/logout"
body: "*"
};
}
```

```yaml
paths:
/api/logout:
post:
requestBody:
content:
application/json: {}
required: true
```

Required empty body causes exceptions on swagger-ui. And I'm not sure which side is the problem.

Is it possible to provide an option to set `requestBody.required`? Like this:

```bash
--openapi_out=required_request_body=true/false/only_nonempty:.
```