Avoid a crash in _quote_params for some default values.

Fix #316.

Author: aaugustin

debug_toolbar/utils/tracking/db.py

@@ -86,6 +86,8 @@ def _quote_expr(self, element):
             return repr(element)
 
     def _quote_params(self, params):
+        if not params:
+            return params
         if isinstance(params, dict):
             return dict((key, self._quote_expr(value))
                             for key, value in params.iteritems())