Consider a thread done if current_work is null

[lyuma]

Fixes #48257
Addresses one of the causes of #49324

I forgot to make a PR for this fix, so here it is finally. I was hoping someone with more knowledge of the threaded importer would make a comment, but it seems to work well for me so far.

Read #48257 for more detail

[akien-mga]

Makes sense to me. Let's get it validated by @reduz but I can see how returning false here would lead to troubles.

[lyuma]

I would also ask that you check the theory in this comment by @fasterthansound:
#48257 (comment)

There could be an underlying issue that causes current_work to be null.

[akien-mga]

There could be an underlying issue that causes current_work to be null.

Definitely, but this change doesn't hide it, it will still throw an error message. So this will need to be fixed anyway eventually, but at least it shouldn't freeze anymore (but there might still be non-working functionality due to this error).

Edit: But indeed if this PR closes #49324, we'll probably want a new issue focused on the fact that current_work is still unexpectedly null and triggers this error condition.

[akien-mga]

Approved by @reduz on RC.

[akien-mga]

Thanks!