Fix error on empty bib files

[SebastianM-C]

Fix #13

[codecov-commenter]

Codecov Report

Merging #15 (b06a43d) into master (e6bbf4e) will decrease coverage by 0.53%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
- Coverage   91.59%   91.05%   -0.54%     
==========================================
  Files           3        3              
  Lines         119      123       +4     
==========================================
+ Hits          109      112       +3     
- Misses         10       11       +1     

Impacted Files	Coverage Δ
src/citations.jl	94.36% <80.00%> (-1.23%)	⬇️
src/tool_report.jl	85.71% <100.00%> (+0.29%)	⬆️

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