This condition may not be the best solution, since get_data will only return `[]`.  I haven't tried, but something like this seems better:
```
def get_data(self):
    obj_id = self.GET['object_id']
    try:
        obj = self.get_queryset().get(pk=obj_id)
    except (self.model.DoesNotExist, ValueError):  # ValueError when obj_id == ''
        label = _('?')
    else:
        label = get_label(obj)
    return [{'value': obj_id, 'label': label}]
```