Grade report should not count an assignment as "graded" if all grades are zero

[DavidWhitlock]

The Canvas export reports a grade of "0.0" for a submission that hasn't been graded yet.

As a result, the mid-term grade report (in which some students had submitted the mob programming reflection and some hadn't yet because it wasn't due) listed some grades as "missing" when, in reality, they hadn't been graded yet.

So, if none of the submissions for an assignment has been graded, then the assignment shouldn't show up in the report.