OpenAPI tweaks and fixes

[bkoelman]

See commit messages for details.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.87%. Comparing base (58cc1cb) to head (d7cba3e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...Core.OpenApi.Swashbuckle/SchemaGenerationTracer.cs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1748      +/-   ##
==========================================
+ Coverage   90.78%   90.87%   +0.09%     
==========================================
  Files         474      475       +1     
  Lines       14697    14690       -7     
  Branches     2319     2319              
==========================================
+ Hits        13342    13350       +8     
+ Misses        921      911      -10     
+ Partials      434      429       -5     

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