fixes #1786 export to piwigo broken with php 8.1

include/ws_functions/pwg.categories.php

@@ -318,7 +318,7 @@ SELECT
     }
 
     $row['comment'] = strip_tags(
-      trigger_change(
+      (string) trigger_change(
         'render_category_description',
         $row['comment'],
         'ws_categories_getList'
@@ -1307,4 +1307,4 @@ SELECT DISTINCT
   return $output;
 }
 
-?>
+?>

include/ws_functions/pwg.images.php

@@ -1803,7 +1803,7 @@ function ws_images_exist($params, $service)
     // search among photos the list of photos already added, based on md5sum list
     $md5sums = preg_split(
       $split_pattern,
-      $params['md5sum_list'],
+      (string) $params['md5sum_list'],
       -1,
       PREG_SPLIT_NO_EMPTY
     );