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Fixes an issue where when a zero value wrench is input to the
`set_external_force_and_torque` function, the force and torque buffer
arrays are not properly updated. Also added unit tests.

Fixes https://github.com/isaac-sim/IsaacLab/issues/262

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- Bug fix (non-breaking change which fixes an issue)

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---------
Signed-off-by: Kelly Guo <kellyg@nvidia.com>
Co-authored-by: Kelly Guo <kellyg@nvidia.com>