[UDisks] Fix auto-mount regression

Summary:
BUG: 389479
Devices with empty path stopped auto-mounting after commit 1384f275.
This diff fixes the regression by checking if the path is empty.

Reviewers: #frameworks, broulik

Reviewed By: broulik

Subscribers: ngraham, sefaeyeoglu, rikmills, #frameworks

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D10671