[2.7] Fix name of '\0'. (GH-14225)

'\0' is the NUL byte not NULL..
(cherry picked from commit 7821b4c6)

Modules/_csv.c

@@ -818,7 +818,7 @@ Reader_iternext(ReaderObj *self)
             if (c == '\0') {
                 Py_DECREF(lineobj);
                 PyErr_Format(error_obj,
-                             "line contains NULL byte");
+                             "line contains NUL");
                 goto err;
             }
             if (parse_process_char(self, c) < 0) {