Set callback as an optional

[mahmedmushtaq]

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• Open Issue ( Issue Number TypeError: callback is not a function google-cloud-node#3525 )
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Fixes googleapis/google-cloud-node#3525 🦕

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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[sofisl]

Hi @mahmedmushtaq, this part of the library is actually autogenerated, and actually lives here: https://github.com/googleapis/gax-nodejs

Feel free to open an issue on that library to discuss!