Type stability cleanup in `epi_archive`

[brookslogan]

Try to keep other_keys always as character class, using character(0L) instead of NULL, and additional_metadata always as a list.

[brookslogan]

• The other_keys part is a very ephemeral thing; it only exists as arguments to archive construction functions before being baked into the DT key, but it's probably still cleaner to make the default for it character(0L) and validate that it's a character vector.
• The additional_metadata does stick around as a field of the archive, so it's a bit more important to do the same steps above... or just remove as a field altogether. People can just use attributes instead.