[csharp-netcore] Moved null checks from constructor to JsonConverter

[devhl-labs]

Moving the null checks to the JsonConverter. Its a bit unusual to have them in the constructor, and not flexible. It is easy for users to replace converters in the event users do not want the null checks. The only reason they are there at all is because System.Text.Json does not have missing members support like Newtonsoft.

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• Read the contribution guidelines.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
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  Commit all changed files.
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[devhl-labs]

The diff is not related.

[wing328]

CI failure (samples outdated) already fixed in the master.