code: func (e *emptyBackend) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
  	if len(e.next.Servers()) == 0 {
  		rw.WriteHeader(http.StatusServiceUnavailable)
  		_, err := rw.Write([]byte(http.StatusText(http.StatusServiceUnavailable)))
  		if err != nil {
  			http.Error(rw, err.Error(), http.StatusInternalServerError)
  		}
  	} else {
  		e.next.ServeHTTP(rw, req)
  	}
  }
docs: // ServeHTTP responds with 503 when there is no active Server and otherwise
  // invokes the next handler in the middleware chain.