More correct use Optional

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
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• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[yorlov]

I signed it!

[googlebot]

CLAs look good, thanks!

[thso]

Thank you for the contribution. Small comment only.