[BUG] [aspnetcore] Generated code produces warnings

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating an aspnetcore server, the output produces several compiler warnings.

openapi-generator version

6.01

Generation Details

openapi-generator-cli generate -i <spec> -g aspnetcore -o <output> --package-name <packageName> --additional-properties=classModifier=abstract --additional-properties=buildTarget=library

Steps to reproduce

There are warnings in Attributes\ValidateModelStateAttribute.cs and Converters\CustomEnumConverter.cs: CS8603: Possible null reference return

Formatters\InputFormatterStream.cs is missing XML-doc comments

Related issues/PRs

Couldn't find any

Suggest a fix

Add XML-doc comments to modules\openapi-generator\src\main\resources\aspnetcore\3.0\Formatters\InputFormatterStream.mustache

Compiler warnings can be resolved by including {>partial_header} at the top of modules\openapi-generator\src\main\resources\aspnetcore\3.0\typeConverter.mustache and modules\openapi-generator\src\main\resources\aspnetcore\3.0\validateModel.mustache.
This adds the <auto-generated> tag as a comment, which causes the compiler to ignore the issues.

I have a fix ready for this, but can't seem to get access to push my code and create a PR.

Start with the ASP.NET Core templates modules/openapi-generator/src/main/resources/aspnetcore/3.0/Formatters/InputFormatterStream.mustache, typeConverter.mustache, and validateModel.mustache. Generate an aspnetcore server using the command shown, then verify that the generated project no longer reports CS8603 warnings and the missing XML-doc comments in InputFormatterStream.cs are addressed.