Do not introduce a npm engine when non existing ⛔

[AdrieanKhisbe]

Currently, running a bump node to a yarn package cause the introduction of a npm entry.

This is not intended and notably breaks heroku deployments.

This was locally tested, running a bump on a project using yarn.
This should be released as a patch

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.08%. Comparing base (f7f4a37) to head (d17ae33).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #152   +/-   ##
=======================================
  Coverage   75.08%   75.08%           
=======================================
  Files          19       19           
  Lines         618      618           
=======================================
  Hits          464      464           
  Misses        154      154           

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