Properly show "/" for %d tab title format

Summary: / path was shown as empty string for %d format.

Test Plan:

• Use %d in tab title format
• Run cd /

Actual result: %d replaced with ``
Expected result: %d replaced with /

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D17866