Force id to be string

See contao-community-alliance/dc-general#342

Author: discordier

src/MetaModels/DataAccess/ItemPersister.php

@@ -191,7 +191,7 @@ private function createNewItem(IItem $item)
             ->set($data)
             ->execute()
             ->insertId;
-        $item->set('id', $itemId);
+        $item->set('id', (string) $itemId);
 
         // Set the variant group equal to the id.
         if ($isNewBaseItem) {