gradebook appears to bomb if there is no data to aggregate

lms/djangoapps/gradebook/models.py

@@ -77,6 +77,7 @@ class StudentGradebook(TimeStampedModel):
                     queryset = queryset.exclude(user__in=exclude_users)
                     gradebook_user_count = len(queryset)
 
+                if gradebook_user_count:
                     # Calculate the class average
                     course_avg = queryset.aggregate(Avg('grade'))['grade__avg']
                     if course_avg is not None and gradebook_user_count < total_user_count:
@@ -88,6 +89,7 @@ class StudentGradebook(TimeStampedModel):
                         data['course_max'] = queryset.aggregate(Max('grade'))['grade__max']
                         data['course_min'] = queryset.aggregate(Min('grade'))['grade__min']
                         data['course_count'] = queryset.aggregate(Count('grade'))['grade__count']
+
                     # Construct the leaderboard as a queryset
                     data['queryset'] = queryset.values(
                         'user__id',