Fixed Single-Language

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

When multilanguage is disabled, so no locales are set in `cmf_core.multilang`, the DIC parameter `cmf_core.multilang.locales` did not exists. The [SimpleCMS's Phpcr RouteProvider is depending on this parameter](https://github.com/symfony-cmf/SimpleCmsBundle/blob/master/Resources/config/routing-phpcr.xml#L28), so it resulted in an error.

Setting a default empty array in the configuration solved this bug.