[PHP] Class 'null' not found

Hello,
I have this error:

Fatal error: Uncaught Error: Class 'null' not found in /Users/mickael/Desktop/swagger-codegen/SwaggerClient-php/lib/ObjectSerializer.php:294
Stack trace:
#0 /Users/mickael/Desktop/swagger-codegen/SwaggerClient-php/lib/ObjectSerializer.php(247): Swagger\Client\ObjectSerializer::deserialize(Object(stdClass), 'null', NULL)
#1 /Users/mickael/Desktop/swagger-codegen/SwaggerClient-php/lib/ObjectSerializer.php(311): Swagger\Client\ObjectSerializer::deserialize(Array, 'null[]', NULL)
#2 /Users/mickael/Desktop/swagger-codegen/SwaggerClient-php/lib/Api/DefaultApi.php(157): Swagger\Client\ObjectSerializer::deserialize(Object(stdClass), '\\Swagger\\Client...', Array)
#3 /Users/mickael/Desktop/swagger-codegen/SwaggerClient-php/lib/Api/DefaultApi.php(98): Swagger\Client\Api\DefaultApi->vaultPostWithHttpInfo(Object(Swagger\Client\Model\VaultRequest))
#4 /Users/mickael/Desktop/swagger-codegen/SwaggerClient-php/test.php(18): Swagger\Client\Api\DefaultApi->vaultPost(Object(S in /Users/mickael/Desktop/swagger-codegen/SwaggerClient-php/lib/ObjectSerializer.php on line 294

Swagger-codegen version

swagger-codegen version: 2.3.1

Swagger declaration file content or url

https://gist.github.com/mickaelmonsieur/6f761d5f9f1077b49fe3f7a95a4a78d0

Command line used for generation

swagger-codegen generate -i /tmp/swagger.json -l php -o /Users/mickael/Desktop/swagger-codegen/

Steps to reproduce

<?php
require_once(__DIR__ . '/vendor/autoload.php');

$apiInstance = new Swagger\Client\Api\DefaultApi(
    // If you want use custom http client, pass your client which implements `GuzzleHttp\ClientInterface`.
    // This is optional, `GuzzleHttp\Client` will be used as default.
    new GuzzleHttp\Client(['cert' => ['../../certs/xxxxxxxxxx.net.pem',
    'PassW0rdXXX']]),
    $config
);

$data = array('source_id' => 40, 'allow_partial_update' => false);
$json_vault_request = new \Swagger\Client\Model\VaultRequest($data); // \Swagger\Client\Model\VaultRequest |

echo $json_vault_request->__toString();

try {
    $result = $apiInstance->vaultPost($json_vault_request);
    print_r($result);
} catch (Exception $e) {
    echo 'Exception when calling DefaultApi->vaultPost: ', $e->getMessage(), PHP_EOL;
}

?>

Start with SwaggerClient-php/lib/ObjectSerializer.php, especially the deserialize calls around lines 247, 294, and 311, and reproduce the failure using the linked declaration and PHP example. Trace how the generated client handles null and null[] response types; done means the reported request no longer raises Class 'null' not found, with regression coverage for the case.