gh-93442: Add test for _Py_CAST(nullptr). (gh-93505) (gh-93509)

(cherry picked from commit 713eb18)

Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>

Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>

Author: miss-islington

Lib/test/_testcppext.cpp

@@ -77,6 +77,9 @@ test_api_casts(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
     // gh-93442: Pass 0 as NULL for PyObject*
     Py_XINCREF(0);
     Py_XDECREF(0);
+    // ensure that nullptr works too
+    Py_XINCREF(nullptr);
+    Py_XDECREF(nullptr);
 
     Py_DECREF(obj);
     Py_RETURN_NONE;