[PHP] Fix parentSchema conditional causing parent call when parent isn't present (#8705)

* Fix parentSchema conditional

we cannot call `parent::` if parent isn't present. Everywhere else in the schema we're checking for `{{parentSchema}}` but here we're checking for `{{parent}}` which is causing errors when parent is not present

* Update samples

samples/client/petstore/php/OpenAPIClient-php/lib/Model/NullableClass.php

@@ -254,7 +254,7 @@ class NullableClass implements ModelInterface, ArrayAccess, \JsonSerializable
      */
     public function listInvalidProperties()
     {
-        $invalidProperties = parent::listInvalidProperties();
+        $invalidProperties = [];
 
         return $invalidProperties;
     }