fix: Null type is now correctly appended as {"type": "null"} to oneOf types

[edgarrmondragon]

This was causing config validation failures in meltano-map-transform: https://github.com/MeltanoLabs/meltano-map-transform/actions/runs/7962941688/job/21737507548?pr=221

[codspeed-hq]

CodSpeed Performance Report

Merging #2252 will not alter performance

_{Comparing edgarrmondragon/fix/one-of-append (157c6f2) with main (5e02dac)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5e02dac) 88.54% compared to head (157c6f2) 88.54%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2252   +/-   ##
=======================================
  Coverage   88.54%   88.54%           
=======================================
  Files          54       54           
  Lines        4724     4724           
  Branches      920      920           
=======================================
  Hits         4183     4183           
  Misses        380      380           
  Partials      161      161           

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