fix(sip-dialout): Fix the successful dialout case when error is null

Signed-off-by: Joas Schilling <coding@schilljs.com>

lib/Service/ParticipantService.php

@@ -1265,7 +1265,7 @@ public function startDialOutRequest(SIPDialOutService $dialOutService, Room $roo
 			throw new \InvalidArgumentException('backend');
 		}
 
-		if ($dialOutResponse->dialOut->error->message) {
+		if ($dialOutResponse->dialOut->error?->message) {
 			throw new DialOutFailedException(
 				$dialOutResponse->dialOut->error->code,
 				$dialOutResponse->dialOut->error->message,