Minor cleanup, return error: null in API success call.

Author: vdboor

polymorphic_tree/admin/parentadmin.py

@@ -198,7 +198,7 @@ def api_node_moved_view(self, request):
         # Report back to client.
         return HttpResponse(simplejson.dumps({
             'action': 'success',
-            'error': '',
+            'error': None,
             'moved_id': moved_id,
             'action_column': self.actions_column(moved),
         }), content_type='application/json')