Nullable response have a weird behavior right now

```cadl
import "@typespec/http";

using TypeSpec.Http;

@error
model Error {
message: string;
}

@route("/1") op read1(): string | null;
@route("/2") op read3(): string | null | Error;
```

[Playground link](https://cadlplayground.z22.web.core.windows.net/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBUeXBlU3BlYy5IdHRwOwoKQGVycm9yCm1vZGVsIEXEDCB7CiAgbWVzc2FnZTogc3RyaW5nOwp9CgpAcm91dGUoIi8xIikgb3AgcmVhZDEoKcgkIHwgbnVsbDvKKDLKKDPRKCB8xm87Cg%3D%3D&e=@typespec/openapi3&options=%7B%7D)

Resulting is a `204` no content before PR #355 which remove ignore `null` responses.

Feel like it should be making the string body nullable