Volume not generating bibtex without <frontmatter>?

[mbollmann]

<frontmatter> is just the special stub for paper 0. If we don't generate it, then no bibtex is generated for the volume itself. We want to generate this volume bibtex even if there is no PDF. (If there is a PDF, we add a <url> tag within frontmatter, as we do for papers.) This is all a separate issue.

Originally posted by @mjpost in #2724 (comment)

This sounds like a bug, and the fix should be made in the code rather than generating <frontmatter> blocks that reference non-existant frontmatter. I know the code is already intended to support proper processing of volumes without frontmatter, as there are comments to that effect.