[BUG] ELM: C# 8 nullable feature creates uncompilable (Maybe OneOf<PContactInfo>)

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
  No, it is the complete swagger.json
• Have you validated the input using an OpenAPI validator (example)?
  Yes
• What's the version of OpenAPI Generator used?
  4.2.0
• Have you search for related issues/PRs?
  Yes.
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

As of C# 8, nullable references types have an anotation '?', and unless it is there, the reference cannot be null.

https://docs.microsoft.com/en-us/dotnet/csharp/tutorials/nullable-reference-types

I have a plain DTO like this,

    public class AccResponse
    {
        // ReSharper disable once NotNullMemberIsNotInitialized
        [NotNull] [Required] public BaseResponse Common { get; set; } = new BaseResponse();
        // public string Id { get; set; }
        [Required] public string Name { get; set; } = "";
        public bool AllowGuest { get; set; }
        public PContactInfo? TechContactInfo { get; set; }
        public PContactInfo? AdminContactInfo { get; set; }
        public bool Blocked { get; set; }
        public bool Disabled { get; set; }
        public int MaximumAllowedNamedUsers { get; set; }
        public string? CreateUserEmailMessage { get; set; }
        public string? CreateUserEmailMessageSubject { get; set; }
    }

and the problem is

    public PContactInfo? TechContactInfo { get; set; }

It is actually exactly the same as

    public PContactInfo TechContactInfo { get; set; }

in C# 7 and earlier, i.e. since there is no [Required], that field can be null.

openapi-generator version

4.2.0

OpenAPI declaration file content or url

Command line used for generation

java -jar openapi-generator-cli-4.2.0.jar generate -g elm -i swagger.json -o src/Swagger/

Steps to reproduce

Just generate and look at the code. There is a warning for another problem,

swagger.zip

[main] INFO o.o.codegen.DefaultGenerator - Model ExcelFormattingBase not generated since it's a free-form object

but you can ignore that. I can simplify those objects.

Suggest a fix

Just make it be treated as if neither ? or [Required] is not there.

Start by running the reported OpenAPI Generator 4.2.0 command with swagger.json or swagger.zip and inspect the generated DTOs, especially the PContactInfo properties. Compare nullable reference annotations with the issue's C# 7 and C# 8 examples. Done means generation no longer produces the reported uncompilable nullable output while preserving the stated Required behavior.