harden curve grades dialog to accept undefined + null.

this fix resolves an issue where the "assign zeroes" function
of curve grades dialog was receiving undefined values for scores
instead of null values.

the function now accepts both null and undefined values.

Change-Id: I56db70316b3a4b4b1a2e0f18a7e38ea2880d355f
Reviewed-on: https://gerrit.instructure.com/12403
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>

app/coffeescripts/gradebook2/CurveGradesDialog.coffee

@@ -68,7 +68,7 @@ define [
       for idx, student of @gradebook.students
         score = student["assignment_#{@assignment.id}"].score
         score = @assignment.points_possible if score > @assignment.points_possible
-        score = 0 if score < 0 or score is null and should_assign_blanks
+        score = 0 if score < 0 or !score? and should_assign_blanks
         users_for_score[parseInt(score, 10)] = users_for_score[parseInt(score, 10)] or []
         users_for_score[parseInt(score, 10)].push [ idx, (score or 0) ]
         scoreCount++