Fix KIO::mkpath with qtbase 5.10 beta 4

Summary:
The latest Qt 5.10 beta includes [1] which breaks a bunch of unit tests,
since `url.setPath("//foo")` will now result in an invalid (empty) QUrl.

This patch fixes the KIO::mkpath() case.

[1]: http://code.qt.io/cgit/qt/qtbase.git/commit/?id=f62768d046528636789f901ac79e2cfa1843a7b7

Test Plan:

* I can now create folders from dolphin and plasma.
* fileundomanagertest and mkpathjobtest no longer fail

Reviewers: #frameworks, dfaure

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D8836