Warnings generated for all citations

[adunning]

Thanks for your wonderful work on this filter. When I run it with pandoc 3.2.1, I receive warnings on all citations whether or not they render correctly. For example, running on the base directory of this repository:

$ pandoc test/input.md --lua-filter=multibib.lua

[WARNING] Citeproc: citation Bae not found
[WARNING] Citeproc: citation Knu86 not found
[WARNING] Citeproc: citation Bel not found
[WARNING] Citeproc: citation Nie72 not found

[adunning]

Thank you looking at this! The only difficulty is that this change also silences warnings for legitimately missing citations. I've realized that your earlier https://github.com/pandoc/lua-filters/tree/master/multiple-bibliographies actually works properly in this respect, but I haven't quite sorted out what it is doing differently.

[tarleb]

Oh, that's not good, then. I'll need to take a closer look.