Fix Error "undefined index: file_target"

Added isset for error:
```
Undefined index: file_target at /var/www/owncloud/lib/private/share/share.php#1911
```
In the original code: https://github.com/owncloud/core/commit/154277ab it was called $fileDependant. I think @schiesbn changed it to checking `file_target` because only file-dependant shares have `file_target` set.

I think this is correct, but we might want check for emptiness to be sure `empty($item['file_target'])` in case it's an empty string.