Robot still saggs if interfaces are not claimed

[christophfroehlich]

Since #177 the joints without claimed interfaces are set to have zero velocity. I'm still not exactly sure if this really solves an issue, because IMHO it does not work but breaks other configurations (passive joints were already fixed; but joints without active controller might be expected to fall down, e.g. with effort interface: fix proposed with #251 ):

• ros2 launch ros2_control_demo_example_9 rrbot_gazebo_classic.launch.py gui:=true
• Change the gravity to be horizontal in gazebo UI
• The robot moves

https://github.com/ros-controls/gazebo_ros2_control/assets/3367244/be3d1751-a2f8-4d3f-91fb-89f5d680c9db

If this is an important usecase, we should save the position at time of initialization or perform_command_mode_switch (if a controller was deactivated later) and set also the position to that value, not only the velocity to zero (similar to #213) https://github.com/ros-controls/gazebo_ros2_control/blob/3c8f5bb4a4cc5eb01240afdbda9094f3ea3263ec/gazebo_ros2_control/src/gazebo_system.cpp#L604-L607