Time: Enable operators with different clock sources if one of the times is zero.

Feature request

Feature description

There should be a possibility to compare a Time object constructed by rclcpp::Time() or rclcpp::Time(0) with a Time object obtained from get_clock()->now() and similar without the possibility of a runtime error to enable checking for uninitialized time.

Implementation considerations

This can be implemented in a simple way by modifying the implementation of the rclcpp::Time class' comparison operators in the corresponding file: https://github.com/ros2/rclcpp/blob/rolling/rclcpp/src/rclcpp/time.cpp. An example implementation of the feature is provided here: https://github.com/matemat13/rclcpp/tree/zero_time_fix.

A possibly related feature request: #2025.