[BUG][SWIFT6] generator doesn't respect x-nullable swagger 2.0 property for $ref attributes

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

x-nullable is ignored when $ref is used on an object property by swift6 generator

openapi-generator version

$ yarn openapi-generator-cli version
7.13.0

OpenAPI declaration file content or url

https://gist.github.com/bogdan/f5fbf0f5940d5540b60ca086beef3fea

Steps to reproduce

openapi-generator-cli generate -i spec.json -g swift6 -o .

Open Source/API/Models/Item.swift.

Actual:

    public var activeListing: Listing

Expected:

    public var activeListing: Listing?

Note that it is only reproducable when x-nullable and $ref are used together:

{
        "active_listing": {
          "x-nullable": true,
          "$ref": "#/definitions/Listing"
        }
}

Reproduce with the linked OpenAPI declaration and openapi-generator-cli generate -i spec.json -g swift6 -o ., then inspect Source/API/Models/Item.swift. Trace the swift6 generator handling of the active_listing property when x-nullable and $ref are combined; done means the generated declaration is public var activeListing: Listing?.