fix server error 500 when save to sent items is null

Author: geisi

src/MicrosoftGraphTransport.php

@@ -34,7 +34,7 @@ protected function doSend(SentMessage $message): void
         $email = MessageConverter::toEmail($message->getOriginalMessage());
         $envelope = $message->getEnvelope();
 
-        $html = $email->getHtmlBody();
+        $html = (string) $email->getHtmlBody();
 
         [$attachments, $html] = $this->prepareAttachments($email, $html);