CallMethod is faster with a NULL third-argument than with an empty format string.

Python/bltinmodule.c

@@ -2050,7 +2050,7 @@ builtin_trunc(PyObject *self, PyObject *number)
         /* XXX: The py3k branch gets better errors for this by using
            _PyType_Lookup(), but since float's mro isn't set in py2.6,
            we just use PyObject_CallMethod here. */
-	return PyObject_CallMethod(number, "__trunc__", "");
+	return PyObject_CallMethod(number, "__trunc__", NULL);
 }
 
 PyDoc_STRVAR(trunc_doc,