standard_layout.html.twig: Do not display the group label if no item in group is available

This PR fixes the bug of displaying empty group in top menu with no items: ![sonata_empty_group_label](https://f.cloud.github.com/assets/960844/1260978/75fe86d8-2c20-11e3-94dd-81e03181712d.jpg)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -