Regression: broken nullability after update to v3.9.0

Commencez par exécuter la reproduction C# fournie via SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0) et comparez les sorties de v3.7.0 et v3.9.0. Suivez la sérialisation des schémas OpenAPI 3.0 pour JsonSchemaType.Null dans additionalProperties et un schéma dérivé de allOf ; le travail est terminé lorsque nullable est préservé dans les deux cas sans modifier la sortie affichée pour les autres champs.

Describe the bug
After updating from v3.7.0 to v3.9.0, the nullable property disappears when writing an OpenAPI 3.0 document.

Repro steps

using Microsoft.OpenApi;

var metaSchema = new OpenApiSchema
{
    Type = JsonSchemaType.Object,
    AdditionalProperties = new OpenApiSchema
    {
        Type = JsonSchemaType.Null
    }
};

var baseSchema = new OpenApiSchema
{
    Type = JsonSchemaType.Object,
    Id = "base",
    Properties = new Dictionary<string, IOpenApiSchema>
    {
        ["id"] = new OpenApiSchema
        {
            Type = JsonSchemaType.String
        }
    }
};

var derivedSchema = new OpenApiSchema
{
    Type = JsonSchemaType.Null,
    AllOf = new List<IOpenApiSchema>
    {
        new OpenApiSchemaReference(baseSchema.Id)
    },
    Properties = new Dictionary<string, IOpenApiSchema>
    {
        ["kind"] = new OpenApiSchema
        {
            Type = JsonSchemaType.String
        }
    }
};

var doc = new OpenApiDocument();
doc.Components ??= new OpenApiComponents();
doc.Components.Schemas ??= new Dictionary<string, IOpenApiSchema>();
doc.Components.Schemas.Add("meta", metaSchema);
doc.Components.Schemas.Add("base", baseSchema);
doc.Components.Schemas.Add("derived", derivedSchema);

var result = await doc.SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0);
Console.WriteLine(result);

Using v3.7.0, this prints:

{
  "openapi": "3.0.4",
  "info": { },
  "paths": { },
  "components": {
    "schemas": {
      "meta": {
        "type": "object",
        "additionalProperties": {
          "nullable": true
        }
      },
      "base": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string"
          }
        }
      },
      "derived": {
        "allOf": [
          {
            "$ref": "#/components/schemas/base"
          }
        ],
        "properties": {
          "kind": {
            "type": "string"
          }
        },
        "nullable": true
      }
    }
  }
}

Using v3.9.0, this prints:

{
  "openapi": "3.0.4",
  "info": { },
  "paths": { },
  "components": {
    "schemas": {
      "meta": {
        "type": "object",
        "additionalProperties": {
          "enum": [
            null
          ]
        }
      },
      "base": {
        "type": "object",
        "properties": {
          "id": {
            "type": "string"
          }
        }
      },
      "derived": {
        "allOf": [
          {
            "$ref": "#/components/schemas/base"
          }
        ],
        "properties": {
          "kind": {
            "type": "string"
          }
        }
      }
    }
  }
}

Screenshots/Code Snippets
View of the diff:

Originally reported at: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/4065