DOMDocument::loadHTML(): Empty string supplied as input

Closes #3194

Merge request studip/studip!3303

lib/models/Courseware/BlockTypes/Text.php

@@ -45,6 +45,7 @@ class Text extends BlockType
     {
         $document = new \DOMDocument();
         $old_libxml_error = libxml_use_internal_errors(true);
+        if (!empty($payload['text'])) {
             $document->loadHTML($payload['text']);
             libxml_use_internal_errors($old_libxml_error);
             $imageElements = $document->getElementsByTagName('img');
@@ -58,6 +59,7 @@ class Text extends BlockType
                     $payload['text'] = str_replace($find, $GLOBALS['ABSOLUTE_URI_STUDIP'], $payload['text']);
                 }
             }
+        }
         $payload['text'] = \Studip\Markup::purifyHtml(\Studip\Markup::markAsHtml($payload['text']));
 
         parent::setPayload($payload);