Fix error in many to one template

Sometimes we got something like this Proxies\__CG__\Acme\AdminBundle\Entity\Sample with id == null as value, if we try to generate url for this object, we got excpetion with message: 
``` 
"Parameter "id" for route "admin_acme_admin_sample_edit" must match "[^/]++" ("" given)
 to generate a corresponding URL." 
``` 
 I fixed this bug.

PS:
I got this error then create list like:
``` php
    protected function configureListFields(ListMapper $list)
    {
        $list->add('city.country');
        $list->add('city');
        $list->addIdentifier('name');

        parent::configureListFields($list);
    }
```