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)

Backport of 66bc0f0 from master
Merge pull request #7834 from owncloud/stable6-quota-usequotaevenwhenfreespaceunknown

[stable6] [backport] Still return quota value when free space is unknown