[Bug]: required nullable enum should include null

[havarnov]

Describe the bug

When adding including a nullable enum it should be added to the schema with nullable: true and null should be included in the options, see https://swagger.io/docs/specification/v3_0/data-models/enums/#nullable-enums.

Expected behavior

An nullable enum should be added to the openapi schema on the following format:

type: string

nullable: true  # <---

enum:

  - asc

  - desc

  - null        # <--- without quotes, i.e. null not "null"

Actual behavior

This is how it's added now

type: string

enum:

  - asc

  - desc

Steps to reproduce

using Microsoft.AspNetCore.Mvc;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();
app.UseSwagger();
app.UseSwaggerUI();
app.Run();

[ApiController]
public class IndexController : ControllerBase
{
    [HttpGet("/index")]
    [ProducesResponseType<IndexResponseModel>(StatusCodes.Status200OK)]
    public IActionResult Index()
    {
        return Ok(new IndexResponseModel()
        {
            Timestamp = null,
            State = null,
        });
    }
}

public class IndexResponseModel
{
    public required DateTimeOffset? Timestamp { get; init; }
    public State? State { get; init; }
}

public enum State
{
    On = 1,
    Off = 2,
}

Exception(s) (if any)

No response

Swashbuckle.AspNetCore version

7.2.0

.NET Version

9.0.101

Anything else?

No response

[github-actions]

This issue is stale because it has been open for 60 days with no activity. It will be automatically closed in 14 days if no further updates are made.