[BUGFIX] Prevent exception in A/B testing module

on empty data installations

Related: #46

Classes/Domain/Repository/AbstractRepository.php

@@ -46,6 +46,9 @@ public function persistAll(): void
      */
     protected function convertIdentifiersToObjects(array $identifiers, string $tableName): array
     {
+        if ($identifiers === []) {
+            return [];
+        }
         $identifierList = implode(',', $identifiers);
         $sql = 'select * from ' . $tableName . ' where uid in (' . $identifierList . ')'
             . 'ORDER BY FIELD(uid, ' . $identifierList . ')';