fix(formanswer): PHP 8.1 compatibility: null passed instead of string

Author: btry

inc/targetchange.class.php

@@ -667,7 +667,7 @@ public function save(PluginFormcreatorFormAnswer $formanswer): ?CommonDBTM {
       ];
       foreach ($changeFields as $changeField) {
          $data[$changeField] = $this->prepareTemplate(
-            $this->fields[$changeField],
+            $this->fields[$changeField] ?? '',
             $formanswer,
             $changeField == 'content' // only content supports rich text
          );