Make it possible to go up to root again

Summary:
If u == "file:///", then u.setPath(u.path() + '/') results in u == "".
Avoid making this transformation for all schemes (thanks @dfaure).

Test Plan:
In a File open dialog, go to a nfts disk root, that results in currentUrl = file:///d//
Before, when I pressed the upCd dir (^) I was redirected to my home directory, after I see the root directory.

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: aacid, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13674