effects: Fix a type mismatch warning in WindowHeap
file:///data/projects/usr/lib/qml/org/kde/kwin/private/effects/WindowHeap.qml:55:9: Unable to assign null to QUuid
file:///data/projects/usr/lib/qml/org/kde/kwin/private/effects/WindowHeap.qml:55:9: Unable to assign null to QUuid