Fix handling for 204 status code with a body (#28479)
This ensures we handle 204 and 304 status codes correctly in API routes.
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Errors have helpful link attached, see `contributing.md`
Fixes: https://github.com/vercel/next.js/issues/28464
