refactor: remove unecessary default value

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

lib/Service/SerialNumberService.php

@@ -40,7 +40,7 @@ public function generateUniqueSerial(
 			try {
 				$this->crlMapper->createCertificate(
 					$serialString,
-					$certificateOwner ?? 'Unknown',
+					$certificateOwner,
 					$engineName,
 					$instanceId,
 					$generation,