Fix IndexError in TemplateResponse

[alex-oleshkevich]

Summary

Fixes #2906 (Index Errors in old-style Jinja2Templates.TemplateResponse)

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

Thanks @alex-oleshkevich 🙏