Moveit Forward Kinematics for Empty Joint State

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One could certainly argue for that, but that´s not the current behavior and likely also won´t be changed, as this would be a significant change for all users that rely on the observed behavior. If you look at the code in kinematics_service_capability.cpp L183, you´ll see that first the current robot state is retrieved and then updated using the robot_state in the request. If the requested robot_state is empty, the current robot state is used instead, which explains the behavior you are seeing.