Avoid inefficient way to call functions without argument

Don't pass "()" format to PyObject_CallXXX() to call a function without
argument: pass NULL as the format string instead. It avoids to have to parse a
string to produce 0 argument.

Modules/_pickle.c

@@ -2873,7 +2873,7 @@ save_dict(PicklerObject *self, PyObject *obj)
         } else {
             _Py_IDENTIFIER(items);
 
-            items = _PyObject_CallMethodId(obj, &PyId_items, "()");
+            items = _PyObject_CallMethodId(obj, &PyId_items, NULL);
             if (items == NULL)
                 goto error;
             iter = PyObject_GetIter(items);