fix: better fix for #334

[RAX7]

This PR contains a:

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

Motivation / Use-Case

Makes possible to use minimizer/generator filename option with unsupported formats or multiple minimizers

Breaking Changes

Implementation should return Promise<null> for unprocessed data now. But worker still returns not nullable Promise<WorkerResult>.

Additional Info

[codecov]

Codecov Report

Base: 87.57% // Head: 87.77% // Increases project coverage by +0.19% 🎉

Coverage data is based on head (f19d8ab) compared to base (667863d).
Patch coverage: 91.42% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   87.57%   87.77%   +0.19%     
==========================================
  Files           4        4              
  Lines         644      638       -6     
  Branches      242      243       +1     
==========================================
- Hits          564      560       -4     
+ Misses         72       70       -2     
  Partials        8        8              

Impacted Files	Coverage Δ
src/utils.js	80.77% <72.72%> (+0.50%)	⬆️
src/worker.js	93.75% <100.00%> (-3.93%)	⬇️

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[alexander-akait]

The good solution, thank you