[Aspnetcore] Fix nullable warnings part 1

[JFCote]

This is part 1 of an ongoing effort to remove most warning from the code generation for ASP .NET Core server.
They are mostly linked to bad handling of nullable.

Technical committee: @mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

PR checklist

• 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.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[devhl-labs]

You can use {{nrt?}} instead of {{#nullableReferenceTypes}}?{{/nullableReferenceTypes}}

[JFCote]

Oh! I didn't know! Should I change my code?

[devhl-labs]

Its the same just shorter.

[wing328]

thanks for the PR. let's give it a try.

[wing328]

samples updated via d8ed253