Use strict undefined

[AdrienVannson]

Summary

This prevents jinja from silencing errors occuring in the template file. I mentioned this here: https://github.com/danielgtaylor/python-betterproto/pull/630

At that time, I thought it would be necessary to fix other bugs before it works, but it actually already works.

Checklist

• [X] If code changes were made then they have been tested.