code: public static DoubleStreamEx of(OptionalDouble optional) {
          return optional.isPresent() ? of(optional.getAsDouble()) : empty();
      }
docs: Returns a sequential {@code DoubleStreamEx} containing an
  {@link OptionalDouble} value, if present, otherwise returns an empty
  {@code DoubleStreamEx}.
  
  @param optional the optional to create a stream of
  @return a stream with an {@code OptionalDouble} value if present,
  otherwise an empty stream
  @since 0.1.1