🐛 Ensure that Optional[list] values work correctly with callbacks

[solesensei]

This pull request refactors the generate_list_convertor function to handle the case when the value parameter is None.

Fixes: #762

[github-actions]

📝 Docs preview for commit 5c9ba8f at: https://d5834c3f.typertiangolo.pages.dev

[github-actions]

📝 Docs preview for commit ade4517 at: https://3a35550f.typertiangolo.pages.dev

[solesensei]

@tiangolo, Hi! Please check this review. It blocks our application from upgrading to 0.10.0+.

[solesensei]

cc: @svlandeg

[svlandeg]

Hi @solesensei, thanks for the PR! We've got this on our internal queue and will let you know once we've been able to review it 🙏

[solesensei]

@svlandeg I'm sorry, but could you please take a look at this PR? It's a simple one-line fix that shouldn't take much time.

Also it's the most voted unresolved issue of all time

[svlandeg]

@solesensei: like I said, we have this on our queue and will look at it soon. Please refrain from pinging maintainers directly, as this makes our Github mentions very difficult to use and we don't just give priority to whoever pings the most 😉