multiple-bibliographies crashes when given a list of bibliography files

[jdutant]

The filter multiple-bibliographies crashes if a bibliography or bibliography_topic field in the metadata contains a list of filenames instead of a single filename.

Versions:

pandoc 2.11.4
Compiled with pandoc-types 1.22, texmath 0.12.1, skylighting 0.10.2,
citeproc 0.3.0.5, ipynb 0.1.0.1

MWE

Take the filter's sample.md and replace the metadata with:

---
title: Multiple Bibliographies Demo
bibliography_sources: primary.bib
bibliography_recommended_reading:
  - primary.bib
  - secondary.bib
nocite: '@Knu86, @Bae'
---

Run the filter, you get an error:

$ pandoc -s -L multiple-bibliographies.lua sample.md -t html
File primary.bib secondary.bib not found in resource path
Error running filter multiple-bibliographies.lua:
PandocFilterError "pandoc" "Filter returned error status 99"

Found while doing tests for https://github.com/pandoc/lua-filters/issues/165.