Fix null examples being represented as the string null

[jgarciadelanoceda]

Fixes #3784.

I think I have found another thing derivated from the OpenAPI.NET upgrate from 1.X to 2.X, previously when there was declared the example as null that example could be shown on the OpenApi document. Now I cannot create a JsonNode from "null" string and the JsonNode not being null so the serializer does not serialize it(A minor thing I believe).

I tried creating a JsonNode from null string many ways but I think it's not possible by design JsonObject.Parse("null")

https://github.com/microsoft/OpenAPI.NET/blob/8bec88013cbb0f86f7f43dfdb039ce83bf83beaf/src/Microsoft.OpenApi/Models/OpenApiSchema.cs#L535

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.93%. Comparing base (756b759) to head (2f72641).
⚠️ Report is 45 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3788      +/-   ##
==========================================
+ Coverage   94.67%   94.93%   +0.25%     
==========================================
  Files         111      111              
  Lines        3868     3886      +18     
  Branches      780      784       +4     
==========================================
+ Hits         3662     3689      +27     
+ Misses        206      197       -9     

Flag	Coverage Δ
Linux	94.93% <100.00%> (+0.25%)	⬆️
Windows	94.93% <100.00%> (+0.25%)	⬆️
macOS	94.93% <100.00%> (+0.25%)	⬆️

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[martincostello]

I tried creating a JsonNode from null string many ways but I think it's not possible by design JsonObject.Parse("null")

I think that's basically why I ended up changing the code that caused the bug in the first place. I couldn't get it to work and assumed that it wouldn't matter.