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/_collectionsmodule.c

@@ -2009,7 +2009,7 @@ defdict_reduce(defdictobject *dd)
         args = PyTuple_Pack(1, dd->default_factory);
     if (args == NULL)
         return NULL;
-    items = _PyObject_CallMethodId((PyObject *)dd, &PyId_items, "()");
+    items = _PyObject_CallMethodId((PyObject *)dd, &PyId_items, NULL);
     if (items == NULL) {
         Py_DECREF(args);
         return NULL;

Modules/_datetimemodule.c

@@ -1378,7 +1378,7 @@ time_time(void)
     if (time != NULL) {
         _Py_IDENTIFIER(time);
 
-        result = _PyObject_CallMethodId(time, &PyId_time, "()");
+        result = _PyObject_CallMethodId(time, &PyId_time, NULL);
         Py_DECREF(time);
     }
     return result;
@@ -2703,7 +2703,7 @@ date_isoformat(PyDateTime_Date *self)
 static PyObject *
 date_str(PyDateTime_Date *self)
 {
-    return _PyObject_CallMethodId((PyObject *)self, &PyId_isoformat, "()");
+    return _PyObject_CallMethodId((PyObject *)self, &PyId_isoformat, NULL);
 }
 
 
@@ -2729,7 +2729,7 @@ date_strftime(PyDateTime_Date *self, PyObject *args, PyObject *kw)
                                       &format))
         return NULL;
 
-    tuple = _PyObject_CallMethodId((PyObject *)self, &PyId_timetuple, "()");
+    tuple = _PyObject_CallMethodId((PyObject *)self, &PyId_timetuple, NULL);
     if (tuple == NULL)
         return NULL;
     result = wrap_strftime((PyObject *)self, format, tuple,
@@ -3675,7 +3675,7 @@ time_repr(PyDateTime_Time *self)
 static PyObject *
 time_str(PyDateTime_Time *self)
 {
-    return _PyObject_CallMethodId((PyObject *)self, &PyId_isoformat, "()");
+    return _PyObject_CallMethodId((PyObject *)self, &PyId_isoformat, NULL);
 }
 
 static PyObject *

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);