Response body retrieval

case.ts

// Determine a body of the given Response.
// In case of a Response that has no body (i.e. 204 response),
// return `null`.
function getResponseBody(res: Response) {
  res.body // ReadableStream, locked
  await res.text() // always "" for a response with no body
}

// Empty string cannot be used as a response body 
// for the responses that have no body (i.e. 204 response).
new Response('', { status: 204 }) // TypeError

// The correct way to construct such response
// would be to pass `null` as its body:
new Response(null, { status: 204 })

// However, there is no way to retrieve that `null` body
// from a given response instance.
getResponseBody(responseWith204StatusCode) // ???