[3.10] gh-97728: Argument Clinic: Fix uninitialized variable in the Py_UNICODE converter (GH-97729) (GH-97760)

It affects function os.system() on Windows and Windows-specific modules
winreg, _winapi, _overlapped, and _msi.
(cherry picked from commit 0ee9619a4cba58730c45e65d22288fadbf7680de)