Dont throw php warning if cookie is null

This will fix php warning error.

```Warning: Invalid argument supplied for foreach() in /var/lib/jenkins/jobs/Behat_docker/workspace/vendor/behat/mink-selenium2-driver/src/Behat/Mink/Driver/Selenium2Driver.php line 460```


var_dump($cookies):

```
array(4) {
        │    'value' =>
        │    NULL
        │    'info' =>
        │    array(26) {
        │      'url' =>
        │      string(43) "HTTP://127.0.0.1:4444/wd/hub/session/cookie"
        │      'content_type' =>
        │      NULL
        │      'http_code' =>
        │      int(0)
        │      'header_size' =>
        │      int(0)
        │      'request_size' =>
        │      int(147)
        │      'filetime' =>
        │      int(-1)
        │      'ssl_verify_result' =>
        │      int(0)
        │      'redirect_count' =>
        │      int(0)
        │      'total_time' =>
        │      double(0.000235)
        │      'namelookup_time' =>
        │      double(1.8E-5)
        │      'connect_time' =>
        │      double(0.000201)
        │      'pretransfer_time' =>
        │      double(0.000228)
        │      'size_upload' =>
        │      double(0)
        │      'size_download' =>
        │      double(0)
        │      'speed_download' =>
        │      double(0)
        │      'speed_upload' =>
        │      double(0)
        │      'download_content_length' =>
        │      double(-1)
        │      'upload_content_length' =>
        │      double(0)
        │      'starttransfer_time' =>
        │      double(0.000231)
        │      'redirect_time' =>
        │      double(0)
        │      'certinfo' =>
        │      array(0) {
        │      }
        │      'primary_ip' =>
        │      string(9) "127.0.0.1"
        │      'primary_port' =>
        │      int(4444)
        │      'local_ip' =>
        │      string(9) "127.0.0.1"
        │      'local_port' =>
        │      int(47763)
        │      'redirect_url' =>
        │      string(0) ""
        │    }
        │    'sessionId' =>
        │    NULL
        │    'sessionUrl' =>
        │    string(43) "HTTP://127.0.0.1:4444/wd/hub/session/cookie"
        │  }
```