Fix protocol selection in KUrlNavigator

Summary:
QUrl no longer accepts an empty authority and a non-empty path that
starts with '//' (Qt commit f62768d046528636789f901ac79e2cfa1843a7b7).
As the result, protocol selection using the menu no longer worked for
all but 'file' scheme.

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10993