Prevent empty 'ServerName', fixes #870

Thanks to h4cc

actions/admin/settings/120.system.php

@@ -59,6 +59,7 @@ return array(
 					'type' => 'string',
 					'default' => '',
 					'save_method' => 'storeSettingHostname',
+					'plausibility_check_method' => 'checkHostname',
 					),
 				'system_froxlordirectlyviahostname' => array(
 					'label' => $lng['serversettings']['froxlordirectlyviahostname'],
@@ -164,4 +165,4 @@ return array(
 		),
 	);
 
-?>
+?>