[4.4] Fix strpos(): Passing null is deprecated (#42773)

* Fix deprecated warning

* Better code.

Co-authored-by: Allon Moritz <allon.moritz@digital-peak.com>

---------

Co-authored-by: Quy <quy@fluxbb.org>
Co-authored-by: Allon Moritz <allon.moritz@digital-peak.com>

plugins/content/fields/src/Extension/Fields.php

@@ -68,7 +68,7 @@ public function onContentPrepare($context, &$item, &$params, $page = 0)
         }
 
         // Prepare the full text
-        if (property_exists($item, 'fulltext') && strpos($item->fulltext, 'field') !== false) {
+        if (!empty($item->fulltext) && strpos($item->fulltext, 'field') !== false) {
             $item->fulltext = $this->prepare($item->fulltext, $context, $item);
         }
     }