Change the export rule for Task/ActualWork.

This patch removes the ActualWork element from the Task element when
the struct.total_spent_hours == 0.

Since the get_scorm_time method return 'PT8H0M0S' when the time argument is nil or zero,
the current implementation generates <ActualWork>PT8H0M0S</ActualWork> if the total_spent_hours equals to 0.