gh-91321: Fix _PyObject_EXTRA_INIT for C++ (#92396)

In C++, the _PyObject_EXTRA_INIT macro now uses nullptr, rather than
0, to initialize the _ob_next and _ob_prev members of the PyObject
structure.

Fix test_cppext failure when Python is built with
./configure --with-trace-refs.

Author: vstinner

Include/object.h

@@ -66,7 +66,7 @@ whose size is determined when the object is allocated.
     PyObject *_ob_next;           \
     PyObject *_ob_prev;
 
-#define _PyObject_EXTRA_INIT 0, 0,
+#define _PyObject_EXTRA_INIT _Py_NULL, _Py_NULL,
 
 #else
 #  define _PyObject_HEAD_EXTRA