[BUG] [csharp-netcore] Invalid expressions are generated using ??

Created by: vaindil

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

The csharp-netcore generator generates invalid C# if an enum is set as required in a definition. With the enum property name present in the "required" object inside the definition of ObjectCreateArgs, the C# below is generated as a constructor for ObjectCreateArgs:

public ObjectCreateArgs(ObjectType objectType = default(ObjectType))
{
    // to ensure "objectType" is required (not null)
    this.ObjectType = objectType ?? throw new ArgumentNullException("snip");;
}

This is invalid C# because objectType is not nullable, therefore the null coalescing operator can't be used with it.

openapi-generator version

4.2.3

OpenAPI declaration file content or url

Click here (rename extension to .json)

Command line used for generation

java -jar openapi-generator-cli.jar generate -i swagger.json -g csharp-netcore -o test-csharp

Steps to reproduce

Run the above command.

Suggest a fix

Enums are not nullable unless declared with ? like other types, so they should be treated as such.