Handle empty first page in PagedDocuments

[nanotone]

Sometimes in the response that PagedDocument gets from a FindPagedDocumentSeriesReferences request, the resulting first page is empty. This causes a NoMethodError when accessing the first document -- first_page.first ends up being nil.

There's probably a bigger question of why requests are being made for zero-document responses, but within this codebase it might be good to return early with something like

return {paging: nil, pages: 1, documents: []} if first_page.empty?

https://sentry.ds.va.gov/department-of-veterans-affairs/efolder/issues/8508/?referrer

[pkarman]

I think we want to distinguish between zero pages and one page with no documents. We want to re-try if there are zero pages, so good to communicate that to the caller. Maybe with an exception?