[csharp-netcore] verbose null checking (!10333) · Merge requests · OpenAPI Tools / openapi-generator

Created by: aeneasr

This patch addresses an issue where csharp-netcore failed to generate compilable code when required attributes were of complex types.

By using the non-shorthand syntax for checking if the value is set, and throwing an error in that case, the compile issues have been resolved.

Read the contribution guidelines.
Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
Run the following to build the project and update samples:
```
./mvnw clean package 
./bin/generate-samples.sh
./bin/utils/export_docs_generators.sh
```
Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
File the PR against the correct branch: master (5.3.0), 6.0.x
If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.