If listbox configured with multiple="true" and checkmark="true", clicking on header checkbox rises error if listbox is empty.

Simple example to reproduce issue:
http://zkfiddle.org/sample/q6qopl/1-empty-listbox

VM1672 zul.sel.wpd:1020 Uncaught TypeError: Cannot read property '$n' of undefined
at init.fireOnSelect (VM1672 zul.sel.wpd:1020)
at init._zkf (VM1672 zul.sel.wpd:936)
at init._doClick (VM1672 zul.sel.wpd:2931)
at HTMLSpanElement.<anonymous> (zk.wpd:18044)
at HTMLSpanElement.dispatch (zk.wpd:5286)
at HTMLSpanElement.elemData.handle (zk.wpd:4957)