Fixes an issue noted at https://github.com/metamath/metamath-knife/pull/130#issuecomment-1735228209 . The whole bibtag processing was skipped when there is no bibliography, meaning that [[ would only get turned into [ if there is a bibliography present which is not great. Now it just treats this the same as an empty or unreadable bibliography: bib tags are linked anyway but a warning is printed.
Fixes an issue noted at https://github.com/metamath/metamath-knife/pull/130#issuecomment-1735228209 . The whole bibtag processing was skipped when there is no bibliography, meaning that
[[would only get turned into[if there is a bibliography present which is not great. Now it just treats this the same as an empty or unreadable bibliography: bib tags are linked anyway but a warning is printed.