Show delete field only if there is a file

Just add some conditions to VichFileType to show delete field only if there is a file.
I think we do need the file_exists call here.
When using with SF 2.5 and Sonata 2.3 (I haven't test with other configuration), I've got an object because of ```getParent()```:
```
$view->vars['object'] = $form->getParent()->getData();
```

If it's a new instance or if the field is not required (i.e the field name is NULL), ```$view->vars['download_uri']``` will still contain the uri prefix (and we don't want to because there is no file).

Or maybe there is something I don't understand.