AbstractApplicationItem: Add null guards to handlers, drop z===undefined
Items may become null on destruction or just because they are really set to null. The Item::z index property can never be undefined, especially on get.
Fixes the following error:
file:///usr/lib/qml/org/kde/kirigami/AbstractApplicationItem.qml:287: TypeError: Value is null and could not be converted to an object
Edited by ivan tkachenko