ZP-1408 Don't log a warning when a user does not enter a subject. Released...

ZP-1408 Don't log a warning when a user does not enter a subject. Released under the Affero GNU General Public License (AGPL) version 3.

src/lib/utils/utils.php

@@ -1361,7 +1361,7 @@ class Utils {
         if (!$rawheaders) {
             return;
         }
-        $message->headers["subject"] = Utils::convertRawHeader2Utf8($rawheaders["subject"], $message->headers["subject"]);
+        $message->headers["subject"] = isset($message->headers["subject"]) ? Utils::convertRawHeader2Utf8($rawheaders["subject"], $message->headers["subject"]) : "";
         $message->headers["from"] = Utils::convertRawHeader2Utf8($rawheaders["from"], $message->headers["from"]);
     }
 }