Legal API: error fetching comments when IA isn't completed

After an IA is filed-and-paid, the user is returned to Filings UI and the IA is auto-expanded in the Filing History list (ledger). The expansion also loads any documents and comments.

If the IA is paid but not yet completed, the documents call returns an empty object (which is fine) but the comments call returns an error.

Reproduce the Filing History ledger expansion for an IA that has been filed and paid but is not completed. Trace the documents and comments requests from the Filings UI through the Legal API, then verify that the comments request no longer errors and behaves consistently with the empty documents response.

Written by the indexing model from the issue text.