Fix EZP-23248 - Notice when accessing eZ Demo frontpage

#### EZP-23248 - Notice when accessing eZ Demo frontpage

Having xdebug enabled and display_errors= on, wheni install an eZ Demo without content package, i have the following notice when i access the eZ Demo frontpage:

```
Notice: Undefined offset: 2 in /var/www/ezpublish5/vendor/ezsystems/demobundle/EzSystems/DemoBundle/Controller/MenuController.php on line 25
```