When assigning short-answer questions on a Quiz, unanswered questions are flagged as unanswered and, if unanswered, are not given an automatic grade of zero for that question.
I can't off-hand think of why an unanswered question would be anything other than a zero, but even if it would make sense to allow for that possibility, there should at least be a setting that would set unanswered questions to be graded automatically as zero rather than only flagged as unanswered.