So there are several pieces in play here.
1. How the gradebook is setup - commonly, it is set to just ignore empty grades. That way grades are not applied to the final grades until the activity is completed.
2. Your quiz setup. I have never seen a grade being entered for a quiz that has not been taken...it should just remain empty and not be included in the final grade. However, you can take care of this by setting a timer on the quiz and then selecting that quizzes are submitted automatically at the end.