Nullable `allOf`s with default `null` values fail validation

[pranjalv123]

Hi,

In OpenAPI 3.0 it's a common pattern to represent a reference that can be null as an allOf with nullable: true.

However, if you set the default to null, openapi-spec-validator attempts to validate null against the referenced schema, and returns an error. The default should take into account the nullability of the schema that it's part of, not just the referenced allOf.

{
  "info": {
    "title": "Example",
    "version": "v1",
  },
  "openapi": "3.0.2",
  "components": {
    "schemas": {
      "FooSchema": {
        "title": "FooSchema",
        "type": "object"
      },
      "BarSchema": {
        "allOf": [
          {
            "$ref": "#/components/schemas/FooSchema"
          }
        ],
        "nullable": true,
        "default": null
      }
    }
  },
  "paths": {}
}

Failed validating 'type' in schema['allOf'][0]:
    {'title': 'FooSchema', 'type': 'object'}

On instance:
    None