Don't return None when queryset is empty (closes #8)

Thanks @vinzd

Author: stephane

webstack_django_sorting/templatetags/sorting_tags.py

@@ -180,6 +180,8 @@ def sort_queryset(self, queryset, ordering):
             else:
                 return queryset.order_by(ordering)
 
+        return queryset
+
     def render(self, context):
         if self.context_var is not None:
             key = self.context_var