fix: don't crash in not parallel mode

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

fixes #394 + update deps

Breaking Changes

No

Additional Info

Hard to tests, but added link to the original issue

[codecov]

Codecov Report

Merging #395 (57165bd) into master (bfbb68a) will increase coverage by 0.02%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
+ Coverage   96.67%   96.69%   +0.02%     
==========================================
  Files           3        3              
  Lines         301      303       +2     
  Branches      130      131       +1     
==========================================
+ Hits          291      293       +2     
  Misses          9        9              
  Partials        1        1              

Impacted Files	Coverage Δ
src/index.js	97.14% <87.50%> (ø)
src/minify.js	95.60% <100.00%> (+0.09%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bfbb68a...57165bd. Read the comment docs.