[FEATURE] Support for OpenApi 3.1 nullable

[frederikstonge]

I migrated my dotnet backend from .NET 8 with Swashbuckle to .NET 10 with Microsoft.AspNetCore.OpenApi.

Some pieces of my schema were referenced as allOf (no idea why) with a nullable attribute.

"advancedSettings": {
  "allOf": [
    {
      "$ref": "#/components/schemas/AdvancedSettings"
    }
  ],
  "nullable": true
},

And now, they generate like this :

"advancedSettings": {
  "oneOf": [
    {
      "type": "null"
    },
    {
      "$ref": "#/components/schemas/AdvancedSettings"
    }
  ]
},

Any idea if there's a way to support oneOf with just type null and the ref without discriminator?