@SuppressWarnings("unchecked")
  public static <T> Call<List<T>> emptyList() {
    return Call.create(Collections.emptyList());
  }
Returns a completed call which has the supplied value. This is useful when input parameters
imply there's no call needed. For example, an empty input might always result in an empty
output.
zipkin/src/main/java/zipkin2/Call.java