code: public static <T> StreamEx<T> of(Optional<? extends T> optional) {
          return optional.isPresent() ? of(optional.get()) : empty();
      }
docs: Returns a sequential {@code StreamEx} containing an {@link Optional}
  value, if present, otherwise returns an empty {@code StreamEx}.
  
  @param <T> the type of stream elements
  @param optional the optional to create a stream of
  @return a stream with an {@code Optional} value if present, otherwise an
  empty stream
  @since 0.1.1