Fix passthrough controller to not read non-existing state_interfaces (backport #1314)

[mergify]

When specifying higher-order splines while no velocity or acceleration state interfaces are configured for the robot, the controller attempted to read from the non-existing interfaces. This PR adds a check whether those interfaces are actually configured before attempting to read from them.

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This is an automatic backport of pull request #1314 done by Mergify.

[mergify]

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Changes to be committed:
	modified:   ur_robot_driver/test/robot_driver.py

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	both modified:   ur_controllers/src/passthrough_trajectory_controller.cpp

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