Sumproduct using blank cells

I have a grade file with this formula in B12.

={IF(SUM(F12:AT12),ROUND(SUMPRODUCT($F$9:$AT$9,F12:AT12),2),"")}

. When I put what percent I want the grade to count in row 9 the formula in B12 uses it to get the raw score even if there is no grade put in as in I12 in the attached file. How can I get it to work for only students that have an actual score on the work?