OGDS Listing: Display empty string for users without a name or firstname.

Before:
![bildschirmfoto 2013-11-21 um 16 14 51](https://f.cloud.github.com/assets/485755/1592675/c3b5620a-52bf-11e3-84bf-0a7b28fe38e4.png)
After:
![bildschirmfoto 2013-11-21 um 16 14 09](https://f.cloud.github.com/assets/485755/1592674/c3b51250-52bf-11e3-88a6-c7063c0c9b4f.png)