Fix optional request bodies

[cubahno]

Operations with requestBody.required: false were failing decode when the client sent no body at all, since the generated handler always called jsonBodyDecoder and treated an empty body as an error.

Now, for optional bodies, the adapter distinguishes runtime.ErrRequestBodyEmpty from other decode errors: an empty body leaves opts.Body nil (so the request reaches the service), while any other decode error still fails as before.
Required bodies are unchanged.