Gradebook calculations when all input grades are null

At the moment, the code for computing calculated grades (The method use_formula in lib/grade/grade_item.php) replaces any missing/null input grades with 0 before applying the formula.

Based on our usage here at the Open University, we would like to propose a small change to this behaviour. What we would like is:
1. If ALL the input grades to a calculation are missing/null, then the result of the calculation is null.
2. Otherwise, the result is calculated as at present, with any missing grades treated as 0.

The attached patch implements this change.

I know everyone is busy with Moodle 2.0 at the moment, but I would be grateful for any consideration you can give this. (The patch should apply to 1.9 or 2.0.)

I am about to make to post in the Gradebook forum to discuss this.