Fixed template loading with empty blocks

[short description]:

__Tasks:__

- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | fixes https://github.com/sulu-cmf/sulu-standard/issues/319#issuecomment-59071989
| BC Breaks     | none
| Doc           | none