Regression: broken nullability after update to v3.9.0

Start by running the supplied C# reproduction through SerializeAsJsonAsync(OpenApiSpecVersion.OpenApi3_0) and compare the v3.7.0 and v3.9.0 outputs. Trace OpenAPI 3.0 schema serialization for JsonSchemaType.Null in additionalProperties and an allOf-derived schema; done means nullable is preserved in both cases without changing the shown output for other fields.

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