Do not require base URL set with Mink

docs/03_mink_integration.md

@@ -20,7 +20,6 @@ default:
     extensions:
         # ...
         Behat\MinkExtension:
-            base_url: 'http://example.com'
             sessions:
                 symfony:
                     symfony: ~

src/Driver/SymfonyDriver.php

@@ -10,7 +10,7 @@ use Symfony\Component\HttpKernel\KernelInterface;
 
 final class SymfonyDriver extends BrowserKitDriver
 {
-    public function __construct(KernelInterface $kernel, string $baseUrl)
+    public function __construct(KernelInterface $kernel, ?string $baseUrl)
     {
         if (!$kernel->getContainer()->has('test.client')) {
             throw new \RuntimeException(sprintf(