To avoid a error window from VTK (6) due to the txtactor having no scalars.

I'm not sure if it is a VTK 6 only - but I guess it shouldn't make VTK 5 fail -
![vtkerror](https://cloud.githubusercontent.com/assets/1165340/3771748/317ec494-18f8-11e4-8291-789f8f7ad173.png)
 anyone who can test?