Sanity-check collection path before performing recursive deletion

Summary:
There is a error paths in directoryForCollection() which might give us
an empty string here, which is then interpreted as the current working
directory.

CCBUG: 414178

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D25346