fix: files with no coverage should be preserved by default

[lqc]

Closes #53

[codecov-io]

Codecov Report

Merging #54 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   97.43%   97.52%   +0.08%     
==========================================
  Files           2        2              
  Lines         117      121       +4     
==========================================
+ Hits          114      118       +4     
  Misses          3        3

Impacted Files	Coverage Δ
src/reporter.js	97.7% <100%> (+0.11%)	⬆️

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[mattlewis92]

Nice job, thank you! Released as 2.0.2 🎉