Fix PHP notice when using indexes without defined types

This fixes a PHP notice that comes up when executing the `fos:elastica:populate` command if one on the indexes in `config.yml` doesn't define any associated type. Example:

```yaml
fos_elastica:
    clients:
        default: { host: %elasticsearch_host%, port: %elasticsearch_port% }
    indexes:
        my_index: ~
        my_index2:
            types:
                # ...
```