[BUGFIX] Compare according to the type

Return-type of `getBodytext()` is string. So the comparison would always be `true`, when comparing to be `!== null`.

Related: https://projekte.in2code.de/issues/

Classes/Mail/ProgressQueue.php

@@ -182,7 +182,7 @@ protected function getSubject(Queue $queue): string
     protected function getBodyText(Queue $queue): string
     {
         $bodytext = $this->parseService->parseBodytext(
-            $queue->getBodytext() !== null ? $queue->getBodytext() : $queue->getNewsletter()->getBodytext(),
+            $queue->getBodytext() !== '' ? $queue->getBodytext() : $queue->getNewsletter()->getBodytext(),
             [
                 'user' => $queue->getUser(),
                 'newsletter' => $queue->getNewsletter(),