Fixed an error in TwigExtension::menu()

In app/config/menu.yml, if you comment or remove all the links from your only one menu : 
```
main:
#  - label: Home
#    path: homepage
```
and if you try to render it : 
```
{{ menu('main', '_sub_menu.twig') }}
```
 Bolt will throw this error : 
```
An exception has been thrown during the rendering of a template ("Invalid argument supplied for foreach()")
```

It's because the twig function menu() in app/src/Bolt/TwigExtension.php doesn't check if the menu retrieved is valid. In case of an empty menu in the config file, $menu will be 'null'.

I assume this is an error which shouldn't happen.

For fixing this error, I added a simple condition which replace $menu with an empty array if it isn't valid. It allow twig to render the template without error.