feat: Nested schema properties can now be defined as nullable

[edgarrmondragon]

Closes https://github.com/meltano/sdk/issues/2487

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📚 Documentation preview 📚: https://meltano-sdk--2488.org.readthedocs.build/en/2488/

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #2488 will not alter performance

_{Comparing edgarrmondragon/feat/nested-nullable-props (37c43f9) with main (4454aee)}

Summary

✅ 6 untouched benchmarks

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.70%. Comparing base (4454aee) to head (37c43f9). Report is 83 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2488   +/-   ##
=======================================
  Coverage   90.70%   90.70%           
=======================================
  Files          62       62           
  Lines        5015     5016    +1     
  Branches      641      641           
=======================================
+ Hits         4549     4550    +1     
  Misses        325      325           
  Partials      141      141           

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