[BUG][php-nextgen] Invalid class name used for nullable parameters (regression)

[frozenbrain]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

We're generating OpenAPI docs from a Symfony project using the NelmioApiDocBundle.

Example endpoint for reference:

public function test(#[MapQueryParameter] ?TestEnum $testParam = null): JsonResponse { /* ... */ }

This generates an endpoint with a parameter that looks like this (full sample linked below):

{
  "name": "testParam",
  "in": "query",
  "schema": {
    "oneOf": [
      { "$ref": "#/components/schemas/TestEnum" },
      { "type": "null" }
    ],
    "default": null
  }
}

Up until 2.17 the generated method looked like this and was working fine (shortened):

/**
 * @param  \OpenAPI\Client\Model\TestEnum|null $test_param test_param (optional)
 */
public function test(
    ?\OpenAPI\Client\Model\TestEnum $test_param = null
): void { /* ... */ }

Since 2.18 the output is broken, as the FQCN is missing the backslashes:

/**
 * @param  \OpenAPIClientModelTestEnum|null $test_param test_param (optional)
 */
public function test(
    ?\OpenAPIClientModelTestEnum $test_param = null
): void { /* ... */ }

The docs are also affected by this, although the example code was already broken (in a different way) with 2.17. Aside from that, at least the client itself worked. For comparison:

// 2.17
$test_param = new \OpenAPI\Client\Model\\OpenAPI\Client\Model\TestEnum(); // \OpenAPI\Client\Model\TestEnum
// ...
/*
| Name | Type | Description  | Notes |
| ------------- | ------------- | ------------- | ------------- |
| **test_param** | [**\OpenAPI\Client\Model\TestEnum**](../Model/.md)|  | [optional] |
*/

// latest
$test_param = new \OpenAPI\Client\Model\\OpenAPIClientModelTestEnum(); // \OpenAPIClientModelTestEnum
// ...
/*
| Name | Type | Description  | Notes |
| ------------- | ------------- | ------------- | ------------- |
| **test_param** | [**\OpenAPIClientModelTestEnum**](../Model/.md)|  | [optional] |
*/

openapi-generator version

Worked from 2.14 to 2.17.
Broken since 2.18.

OpenAPI declaration file content or url

https://gist.github.com/frozenbrain/8f36f90d00ca61a3b7056726ce80514e

Generation Details

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:latest generate --generator-name php-nextgen --input-spec /local/openapi.json --output /local/client-latest

Steps to reproduce

Generate a client using the command above from the provided openapi.json file.