[HOTFIX] Fixed page URL fetching for internal links used in snippets

Fixed page URL fetching for internal links used in snippets

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu/issues/541
| BC Breaks     | no
| Doc           | none
@chirimoya sure that there cannot be a null value for url? because backend returns all locales with null values if no translation in this language!