code: public static <U> CompletionStage<U> completedStage(U value) {
      return new MinimalStage<U>((value == null) ? NIL : value);
    }
docs: Returns a new CompletionStage that is already completed with the given value and supports only
  those methods in interface {@link CompletionStage}.
  
  @param value the value
  @param <U> the type of the value
  @return the completed CompletionStage
  @since 9