Exclude 0-point quizzes from total grade calculation.

### Reproduction Steps

1. Create a new quiz inside a new course inside a new lesson.
2. Add a single question on that quiz. Make sure that that question has a **non-zero point** (_e.g._, 1 point)
3. Create another quiz inside the same course on a different lesson. Make sure that that question has a **zero points** (_i.e._, 0 point)
4. Enroll to the course.
5. Take the first quiz and pass it so that your grade is 100%.
6. Take the second quiz.

### Expected Behavior
The total grade should still be 100%.

### Actual Behavior
The total grade should still be 50%.

This issue has be recreated:
+ [x] Locally
+ [ ] On a staging site
+ [x] On a production website
+ [x] With only LifterLMS and a default theme