fix deprecated creation of dynamic property

caught by Scrutinizer after upgrading from PHP 7.4 to 8.2

tst/Data/GoogleCloudStorageTest.php

@@ -9,6 +9,7 @@ class GoogleCloudStorageTest extends TestCase
 {
     private static $_client;
     private static $_bucket;
+    private $_model;
 
     public static function setUpBeforeClass(): void
     {