Receive a grade completion condition status should still be returned if custom conditions are not defined

Currently, completion_info::get_other_cm_completion_data fetches the grade completion state, but if custom completion conditions are not configured for a module, the grade completion data is not returned (instead, an empty array is returned). This means any modules which only have view and grade completion conditions (such as Workshop) can never show that the "receive a grade" conditions has been completed.