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