fix: PHP 8.1 deprecation issue (laravel#1205)

laravel.WARNING: trim(): Passing null to parameter laravel#1 ($string) of type string is deprecated in /srv/api/vendor/laravel/lumen-framework/src/Routing/Router.php on line 151

src/Routing/Router.php

@@ -148,7 +148,7 @@ protected static function formatGroupPrefix($new, $old)
         $oldPrefix = $old['prefix'] ?? null;
 
         if (isset($new['prefix'])) {
-            return trim($oldPrefix, '/').'/'.trim($new['prefix'], '/');
+            return trim($oldPrefix ?? '', '/').'/'.trim($new['prefix'], '/');
         }
 
         return $oldPrefix;