[BUG] nullable enums do not add null to the enum list

[Pizzaface]

Describe the bug
When adding a nullable enum Schema, the operation will fail unless null is directly provided in the enum list. This is because nullable is not processed before the schema is validated and not having null in the values causes an error.

Schema Example:

The following causes the Exception, whereas including null will satisfy the schema.

{
  "name": "sortOrder",
  "in": "query",
  "description": "Sort direction. Example: \"asc\".",
  "schema": {
    "type": "string",
    "nullable": true,
    "default": null,
    "enum": [
      "asc",
      "desc"
    ]
  }
}

Corrected:

{
  "name": "sortOrder",
  "in": "query",
  "description": "Sort direction. Example: \"asc\".",
  "schema": {
    "type": "string",
    "nullable": true,
    "default": null,
    "enum": [
      null,
      "asc",
      "desc"
    ]
  }
}

Exception:

("None is not one of ['asc', 'desc']", 'enum', deque([]), None, [], ['asc', 'desc'], None, {'type': 'string', 'nullable': True, 'default': None, 'enum': ['asc', 'desc']}, deque(['enum']), None)

To Reproduce
Steps to reproduce the behavior:

1. Create an endpoint with a nullable string parameter.
2. Do not include null in the enum values
3. Attempt to parse the spec

Expected behavior
When adding a nullable field - the property should have null automatically added to the enum list - with the default being null if not supplied while allowing a set value.

System details (please complete the following information):

• OS: [Ubuntu]
• OpenAPI / Swagger version [OpenAPI 3.0.1]
• Python version [3.10.12]