Fix some PHP 8 warnings

Author: onli

plugins/serendipity_event_textile/serendipity_event_textile.php

@@ -9,6 +9,7 @@
 class serendipity_event_textile extends serendipity_event
 {
     var $title = PLUGIN_EVENT_TEXTILE_NAME;
+    var $markup_elements = [];
 
     function introspect(&$propbag)
     {
@@ -149,7 +150,7 @@ function event_hook($event, &$bag, &$eventData, $addData = null)
 
                             $blocks = array();
                             foreach($preserve_tags as $tag) {
-                                if (preg_match_all('/(<'.$tag.'[^>]?>.*<\/'.$tag.'>)/msU', $eventData[$element] ?? null, $matches )) {
+                                if (preg_match_all('/(<'.$tag.'[^>]?>.*<\/'.$tag.'>)/msU', $eventData[$element] ?? '', $matches )) {
                                     foreach($matches[1] as $match) {
                                         $blocks[] = $match;
                                     }