[BUG] Description

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) No

Description

I have an openapi schema that is autogenerated by @nestjs/swagger library. Unfortunately, and due to strange workplace conditions, I have to generate a C# client. The problem I am running into is every model, regardless of what the annotations specify, will insert non-null validation onto string fields. This happens regardless of:

1. Whether I mark the api property nullable or not.
2. Whether I mark the api property required or not.
3. Whether I specify the runtime argument --additional-properties=validatable=false or not
4. Whether I specify the runtime argument --additional-properties=nullableReferenceTypes=true or not

openapi-generator version

6.6.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.0",
  "components": { "schemas": {
      "CreateFieldDto": {
        "type": "object",
        "properties": {
          "key": { "type": "string" },
          "value": { "type": "string", "nullable": true }
        },
        "required": [ "key", "value" ]
  } } }
// ...
}

Generation Details

/// Generated file: ./src/Org.OpenAPITools/Model/CreateFieldDto.cs
// ...
public CreateFieldDto(string key = default(string), string value = default(string))
        {
            // to ensure "key" is required (not null)
            if (key == null)
            {
                throw new ArgumentNullException("key is a required property for CreateFieldDto and cannot be null");
            }
            this.Key = key;
            // to ensure "value" is required (not null)
            if (value == null)
            {
                throw new ArgumentNullException("value is a required property for CreateFieldDto and cannot be null");
            }
            this.Value = value;
        }
// ...

Steps to reproduce

openapi-generator generate \
 -i <wherever you bake the file with the model>
 -o <wherever you want> \
 -g csharp-netcore \
 --artifact-version 1.0.0 \
 --additional-properties=validatable=false,nullableReferenceTypes=true

Related issues/PRs

Suggest a fix

Start with the provided OpenAPI schema and the openapi-generator generate command using the csharp-netcore generator, then inspect the generated ./src/Org.OpenAPITools/Model/CreateFieldDto.cs. Reproduce the constructor output with version 6.6.0 and the listed additional properties. Done means the generated validation distinguishes the shown required and nullable string properties as described by the schema.