"sort by year" makes papers vanish

### Confirm that this is a bug report

- [X] I want to report an issue that does _not_ concern paper or author metadata.
- [X] I have searched for similar existing issues first.

### Problem Description

The following query gets "about 28" hits, including many paper PDFs. But when I select "Sort by Year of Publication," only 1 hit remains. Maybe this means that publication year is missing for many papers? (Should they be shown anyway, perhaps at the end of the listing?)

https://aclanthology.org/search/?q=snowdon+nun

> I want to report an issue that does not concern paper or author metadata.

Ok, it does concern those things, but not for a _specific_ paper ...

I'm using Google Chrome 114.0.5735.198 on Linux Mint.


Start by reproducing the behavior at https://aclanthology.org/search/?q=snowdon+nun, comparing the default results with the "Sort by Year of Publication" view. Trace the search and sorting entry points from that page; done means matching papers do not disappear merely because publication year is missing, with the handling of undated papers matching the agreed behavior.

索引モデルが issue の本文から書いたものです。