Sorting by Average Progress filters out any courses with 0% completion

### Reproduction Steps

1. Go to LifterLMS > Reporting > Courses, ensure there's a course with 0% progress and a course with > 0%
2. Click on the "Average Progress" header to sort by average progress

### Expected Behavior

Courses are sorted by average progress with 0% progress on the bottom (or top, depending on direction)

### Actual Behavior

Courses with 0% average progress are removed from the table

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