[BUGFIX] Wrong $host value when transport_smtp_server is not set.

Set $host to empty string
if $mailSettings['transport_smtp_server'] does not exist.

Resolves: #84464
Releases: master
Change-Id: I3851bf262e51639e4572dfd8c1967e58b67ca792
Reviewed-on: https://review.typo3.org/56345
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/core/Classes/Mail/TransportFactory.php

@@ -58,7 +58,7 @@ public function get(array $mailSettings): \Swift_Transport
                     $host = $parts[0];
                     $port = $parts[1] ?? null;
                 } else {
-                    $host = (string)$mailSettings['transport_smtp_server'] ?? null;
+                    $host = (string)$mailSettings['transport_smtp_server'] ?? '';
                     $port = null;
                 }