Small fix to remove a warning during the indexing of entities with no created_at attribute (#1586)

Author: damcou

Helper/AlgoliaHelper.php

@@ -593,7 +593,9 @@ protected function prepareRecords(array &$objects, string $indexName): void
         foreach ($objects as $key => &$object) {
             $object['algoliaLastUpdateAtCET'] = $currentCET;
             // Convert created_at to UTC timestamp
-            $object['created_at'] = strtotime($object['created_at']);
+            if (isset($object['created_at'])) {
+                $object['created_at'] = strtotime($object['created_at']);
+            }
 
             $previousObject = $object;