fix: do not show log message when defaultValue returns undefined

[kirillku]

Do not show log message when defaultValue returns undefined because in this case no translation key is added

Fixes #245

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (e5eade7) 90.94% compared to head (ac0282d) 90.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
+ Coverage   90.94%   90.95%   +0.01%     
==========================================
  Files           6        6              
  Lines         563      564       +1     
==========================================
+ Hits          512      513       +1     
  Misses         51       51              

Impacted Files	Coverage Δ
src/parser.js	90.23% <100.00%> (+0.02%)	⬆️

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