Still return quota value when free space is unknown

Fixed the quota storage wrapper to correctly return the quota value when
the free space is not known (which usually happens when the disk_free_space
function is disabled)

Fixes #7553 

To test this, set this in php.ini:
```
disable_functions = disk_free_space
```
and check that the quota still works and appears correctly.

Please review @karlitschek @icewind1991 @schiesbn