Added missing definition checks

Calling the twig function `knp_menu_render` without the parameter `nav_type` throws the following error:
```
Item "nav_type" for "Array" does not exist in
BraincraftedBootstrapBundle:Menu:menu.html.twig at line 111
```
The is also reported in #13. I 've added the missing definition checks.

This is my first pull request. Please let me know if I'm doing something wrong here :)