sftp: fix free space query for urls with empty path

Summary:
e.g. sftp://localhost has an empty path

fall back to assume it means '/'

Test Plan: free space correctly listed on sftp://localhost

Reviewers: ngraham, meven

Reviewed By: meven

Subscribers: kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D27835