(legacy) fix "enableserial" marker in default config system_advanced_admin.php

(cherry picked from commit 9fdca300)

src/www/system_advanced_admin.php

@@ -48,7 +48,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
     $pconfig['nohttpreferercheck'] = isset($config['system']['webgui']['nohttpreferercheck']);
     $pconfig['loginautocomplete'] = isset($config['system']['webgui']['loginautocomplete']);
     $pconfig['althostnames'] = $config['system']['webgui']['althostnames'];
-    $pconfig['enableserial'] = $config['system']['enableserial'];
+    $pconfig['enableserial'] = isset($config['system']['enableserial']);
     $pconfig['serialspeed'] = $config['system']['serialspeed'];
     $pconfig['primaryconsole'] = $config['system']['primaryconsole'];
     $pconfig['enablesshd'] = $config['system']['ssh']['enabled'];