Auto Grade is marking unanswered MCQ submissions as correct

[Imran92]

Steps to Reproduce

1. Create a Course with a lesson with a quiz
2. Add one or more MCQ questions in the Quiz
3. Now take the quiz as a student, submit the quiz without answering any of the questions
4. Now go to SenseiLMS -> Grading and open the quiz submission
5. Click on the "Auto grade" button

What I Expected

To have the unanswered questions marked zero

What Happened Instead

The unanswered questions are graded full marks as correct

PHP / WordPress / Sensei LMS version

8.1, 6.4.1, 4.19.2

Browser / OS version

Chrome, Edge, OperaGx

Screenshot / Video

Quiz

#### Grading

https://github.com/Automattic/sensei/assets/6820724/12d171c3-ecc7-4e4f-bc78-7f785e64c4bb