PHP 8.1 > Always pass string to rtrim

Fixes the following deprecation:
rtrim(): Passing null to parameter FriendsOfBehat#1 ($string) of type string is deprecated in vendor/friends-of-behat/mink-extension/src/Behat/MinkExtension/Context/RawMinkContext.php line 144

src/Behat/MinkExtension/Context/RawMinkContext.php

@@ -141,7 +141,7 @@ public function visitPath($path, $sessionName = null)
      */
     public function locatePath($path)
     {
-        $startUrl = rtrim($this->getMinkParameter('base_url'), '/') . '/';
+        $startUrl = rtrim($this->getMinkParameter('base_url') ?? '', '/') . '/';
 
         return 0 !== strpos($path, 'http') ? $startUrl . ltrim($path, '/') : $path;
     }