[3.8] bpo-37206: Unrepresentable default values no longer represented as None. (GH-13933) (GH-16141)

In ArgumentClinic, value "NULL" should now be used only for unrepresentable default values
(like in the optional third parameter of getattr). "None" should be used if None is accepted
as argument and passing None has the same effect as not passing the argument at all.
(cherry picked from commit 279f44678c8b84a183f9eeb85e0b086228154497)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Python/clinic/sysmodule.c.h b/Python/clinic/sysmodule.c.h
index 93bc332..d2d1503 100644
--- a/Python/clinic/sysmodule.c.h
+++ b/Python/clinic/sysmodule.c.h
@@ -135,7 +135,7 @@
 sys_exit(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
-    PyObject *status = NULL;
+    PyObject *status = Py_None;
 
     if (!_PyArg_CheckPositional("exit", nargs, 0, 1)) {
         goto exit;
@@ -1088,4 +1088,4 @@
 #ifndef SYS_GETANDROIDAPILEVEL_METHODDEF
     #define SYS_GETANDROIDAPILEVEL_METHODDEF
 #endif /* !defined(SYS_GETANDROIDAPILEVEL_METHODDEF) */
-/*[clinic end generated code: output=092edc868de055a6 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=273f9cec8bfcab91 input=a9049054013a1b77]*/