[JsonFormat] Remove obsolete FIXME

We don't need to convert fields that are NULL, since
json_encode will correctly encode those fields.

formats/JsonFormat.php

@@ -6,7 +6,6 @@
 class JsonFormat extends FormatAbstract{
 
     public function stringify(){
-        // FIXME : sometime content can be null, transform to empty string
         $items = $this->getItems();
 
         return json_encode($items, JSON_PRETTY_PRINT);