return Py_None as default for keyword variables

Author: timfel

graalpython/com.oracle.graal.python.cext/src/modsupport.c

@@ -49,8 +49,8 @@ PyObject* get_arg_or_kw(PyObject* argv, PyObject* kwds, char** kwdnames, int arg
     }
     const char* kwdname = kwdnames[argnum];
     void* kwarg = PyDict_GetItem(kwds, to_sulong(truffle_read_string(kwdname)));
-    if (kwarg == Py_None) {
-        return NULL;
+    if (kwarg == NULL) {
+        return Py_None;
     } else {
         return kwarg;
     }