`ros2_control` configuration is not checked for `joint.parameters["device_name"]` tag presence

[roncapat]

https://github.com/ros-industrial/ros2_canopen/blob/7687f3276107d81f52e1f961566942508eb07d85/canopen_ros2_control/src/robot_system.cpp#L94-L95

If an user forgets for example:

      <joint name="right_wheel_joint">
        <param name="device_name">right_wheel_joint</param>   <-- fogets this line

the device_name becomes an empty string. Which in turn makes #241 happen. I suggest to check for non-emptyness of the string.