fix(targetchange): may return null instead of string

Signed-off-by: Thierry Bugier <tbugier@teclib.com>

inc/targetchange.class.php

@@ -752,6 +752,7 @@ public function save(PluginFormcreatorFormAnswer $formanswer) {
             $formanswer,
             $changeField == 'content' // only content supports rich text
          );
+         $data[$changeField] = $data[$changeField] ?? '';
 
          $data[$changeField] = $formanswer->parseTags($data[$changeField], $this, $changeField == 'content');
       }