Bugfix: show `--` as file-size when the folder size is not yet computed

When the folder size is unknown (not yet calculated or cannot be calculated) the server returns `-1` as the size. In this case we now show `--` and not only an empty string

https://github.com/owncloud/owncloud-design-system/issues/1402
https://github.com/owncloud/owncloud-design-system/pull/1393
