fix grade calculations if an assignment group has 0 points possible

This was causing all students in the course to have final grades of null
(NaN).

Change-Id: I2bc268926579ce8c129a8c997f683a9e9097bffc
Reviewed-on: https://gerrit.instructure.com/3016
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Brian Palmer <brianp@instructure.com>

lib/grade_calculator.rb

@@ -140,7 +140,8 @@ def create_group_sums(submissions, ignore_ungraded=true)
 
       # Calculate the tally for this group
       sums[:group_weight] = group.group_weight || 0
-      sums[:tally] = sums[:user_points].to_f / sums[:total_points].to_f rescue 0
+      sums[:tally] = sums[:user_points].to_f / sums[:total_points].to_f
+      sums[:tally] = 0.0 unless sums[:tally].finite?
       sums[:weighted_tally] = sums[:tally] * sums[:group_weight].to_f
       group_sums[group.id] = sums
     end
@@ -177,4 +178,4 @@ def calculate_total_from_group_scores(group_sums, ignore_ungraded=true)
     end
     score
   end
-end
+end