bpo-46712: share more global strings in deepfreeze (gh-32152)

(for gh-90868)

Objects/unicodeobject.c

@@ -258,6 +258,7 @@ static int unicode_is_singleton(PyObject *unicode);
 // Return a borrowed reference to the empty string singleton.
 static inline PyObject* unicode_get_empty(void)
 {
+    _Py_DECLARE_STR(empty, "");
     return &_Py_STR(empty);
 }