Should rclcpp::Parameter throw if trying to stringify an unset parameter?

This is a follow-up issue to a comment left by @clalancette over at https://github.com/ros2/rclcpp/pull/1344.

Currently rclcpp::Parameter will convert an unset parameter into a string if requested (either by `to_string` or using the `<<` operator). Should this be expected or should it throw some sort of exception?
