refactor(yaml): flatten stringifyNode()

[timreichen]

This PR flattens stringifyNode() by using early returns for better readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.13%. Comparing base (6ee055f) to head (d7e90af).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6906      +/-   ##
==========================================
- Coverage   94.14%   94.13%   -0.01%     
==========================================
  Files         582      582              
  Lines       42750    42744       -6     
  Branches     6807     6806       -1     
==========================================
- Hits        40245    40238       -7     
- Misses       2455     2456       +1     
  Partials       50       50              

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