[http-server-csharp] OkResponse is generated as a model

[ArcturusZhang]

We have an operation that just returns 200 without body content:

op hello(): OkResponse;

usually we write this in a way that the op returns void, but because of this issue , I cannot do it.

In the generation result, OkResponse is generated as an empty model, this should not happen especially the operation with OkResponse
is generated as this without a return body type:

public interface ISayHi
{
    Task HelloAsync();
}