ResponseEncode should not write any data when it encode nil

[Donglong]

What happened:

When I don't need to return data to client

1. When using the following code, the DefaultResponseEncode will report an error(http: request method or response status code does not allow body)

func Handle(ctx http.Context) error {
    return ctx.Result(204, nil)
}

2. but, when using the following code, will return null string to client

func Handle(ctx http.Context) error {
    return ctx.Result(200, nil)
}

What you expected to happen:

when call ctx.Result(xxx, nil), server don't return any data to client

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Kratos version (use kratos -v): kratos version v2.2.1
• Go version (use go version):
• OS (e.g: cat /etc/os-release):
• Others: