def METHOD_NAME(self, trajectory_data):
    """Test the `get_step_data` method when some arrays are not defined."""
    trajectory = TrajectoryData()
    trajectory.set_trajectory(symbols=trajectory_data['symbols'], positions=trajectory_data['positions'])
    stepid, time, cell, symbols, positions, velocities = trajectory.get_step_data(3)
    assert stepid == 3
    assert time is None
    assert cell is None
    assert np.array_equal(symbols, trajectory_data['symbols'])
    assert np.array_equal(positions, trajectory_data['positions'][3, :, :])
    assert velocities is None