Users not able to select root category as parent

At the moment there is a bug with creating a new category in certain ACL setups. This prevents users to select "No parent" for new categories, so they can only assign it as child to one of the existing ones.

This pull request fix this issue. 

## Test instructions:
- create a new user group
- allow "backend access" for this group in the global configuration permissions
- allow "Access Administration Interface" and "Create" action for the Article Manager for the group
- create test user, assign to created group
- login with test user
- create new category, notice not possible to select "No parent" as parent
- apply patch
- create new category, and notice that you are now able to select "No parent" as parent

## Screenshots before & after patch
Before:
![screen shot 2014-04-01 at 22 59 49](https://cloud.githubusercontent.com/assets/522834/2584612/9def7410-b9e0-11e3-8ce0-de51b424e751.png)
After:
![screen shot 2014-04-01 at 22 45 25](https://cloud.githubusercontent.com/assets/522834/2584617/a6f8e424-b9e0-11e3-9b83-9c1e71cc057d.png)