Hi there! For Quizzes, any unanswered question gets zero points automatically, even if it is an essay response. So there isn't a need to do anything extra on the instructor side of things. However, maybe I'm not understanding your question; if so, please provide some additional information so folks can help answer!
Thank you for clarifying @Ms_Lawless! On the student side, it will show that they have received zero points for the question, but in Speedgrader it does show the dashes since it is something that is manually graded by the instructor. Unfortunately I am not aware of a way for Speedgrader to change those to zeros for unanswered questions, but I think that would be a great recommendation to Canvas for something they could improve. Consider recommending the idea to Instructure!