gh-115432: Add critical section variant that handles a NULL object (#115433)

gh-115432: Add critical section variant that handles a NULL object (#115433)

This adds `Py_XBEGIN_CRITICAL_SECTION` and
`Py_XEND_CRITICAL_SECTION`, which accept a possibly NULL object as an
argument. If the argument is NULL, then nothing is locked or unlocked.
Otherwise, they behave like `Py_BEGIN/END_CRITICAL_SECTION`.