Before this change if the server sent us xml like this
```
<D:propstat>
<D:prop>
<g0:quota-available-bytes/>
<g0:quota-used-bytes/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
```
Rclone would read the empty XML items as containing 0
After this fix we make sure that we have a value before using it.