[BUGFIX] Fallback to Shadow URL if URL not set

[short description]:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   |  yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/252
| BC Breaks     | NA
Haven't you already created an issue? Maybe you can link to this line, so that we find it more easily later.