tools: fix make-v8.sh

V8's `tools/dev/v8gen.py` does not like being passed an empty string
(`""`).

PR-URL: https://github.com/nodejs/node/pull/36594
Refs: https://github.com/nodejs/node/pull/36099


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>