[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: {F8323417}
After: {F8323416}

Reviewers: ngraham, broulik, #vdg, #plasma

Reviewed By: ngraham, #vdg

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29783