Book Collection without traditional books breaks conversion process

[FyreByrd]

If one of the book collections in an SAB project has no traditional books (e.g. it only has quizzes), then the conversion process will fail during the convert books step.

The failure happens because the Proskomma instance for the book collection does not contain any documents, and thus does not return usable information when queried during the catalog creation process.

PR #580 attempts to address this issue, but potentially causes much bigger problems elsewhere.

Fixing this issue may require a fix in SAB to disallow collections without traditional books, unless that is a valid use case.