[kcm/kwinrules] Fix placeholder message in RulesEditor
Summary:
Fix the anchoring of the placeholder message after https://phabricator.kde.org/R108:9e5498238f88ece28f4f0d22da00879280cb7955
It needs to center on the free space, not the whole ListView.
Test Plan
Before:
After:
Reviewers: ngraham, broulik, VDG, Plasma
Reviewed By: ngraham, VDG
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D29783