public_inbox: Fix email threads without an available root message

[mripard]

Hi,

@stefano-garzarella and @javierm have been reporting on the public-inbox plugin https://github.com/psss/did/pull/329 that they were having crashes.

It turns out that lore doesn't always manage to archive all the messages and we might find emails with a valid in-reply-to header, but can't retrieve the message from the ML archive.

This was partly due to an improper type annotation, so this PR fixes both the type annotations and the actual bug they were both facing.

[stefano-garzarella]

Thanks! Tested with my setup and this PR fixes the issue!

[javierm]

Hi, @mripard :smile:.

I'm afraid I haven't reported anything; don't even know this project :thinking:. Wrong username, maybe?

[mripard]

Yeah, sorry, I meant @martinezjavier

[sandrobonazzola]

Can you please rebase?