Return null on example, as it was with OpenApiNull before

[jgarciadelanoceda]

Ends fixing again #3784 .. I saw there was a thing a bit hidden on Microsoft.OpenApi that allows us to serialize the null as we were doing before with OpenApiNull

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.93%. Comparing base (dc616c9) to head (1e8ddf8).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3793   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files         111      111           
  Lines        3886     3886           
  Branches      784      784           
=======================================
  Hits         3689     3689           
  Misses        197      197           

Flag	Coverage Δ
Linux	94.93% <100.00%> (ø)
Windows	94.93% <100.00%> (ø)
macOS	94.93% <100.00%> (ø)

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[jgarciadelanoceda]

@martincostello what are your thoughts about this one? I found it while answering the comment.. it's a bit nitch but in previous versions <10 Swashbuckle was serializing null in the example