Remove Notice in case a group has no fields

Civi/DataProcessor/Config/Config.php

@@ -23,7 +23,7 @@ class Config extends Container {
    */
   public function getCustomFieldsOfCustomGroup($custom_group_id) {
     $customFieldsPerGroup = $this->getParameter('custom_fields_per_group');
-    return $customFieldsPerGroup[$custom_group_id];
+    return $customFieldsPerGroup[$custom_group_id] ?? [];
   }
 
   /**