We are using zk6 MVVM binding to validate and save a form.
In OurVM we have:
...
private Form fx = new SimpleForm();
private OurBeanClass selected;
...
(setters and getters and other methods)

In our zul file:
<window form="@id('fx') @init(vm.fx) @load(vm.selected) @save(vm.selected, before='updateMetadata')"
apply="org.zkoss.bind.BindComposer" viewModel="@id('vm') @init('our.package.vm.OurVM')"
validationMessages="@id('vmsgs')" >
...
<button label="Save" onClick="@command('updateMetadata')" disabled="@load(not fxStatus.dirty)"/>
...
<intbox id="ourintbox" value="@bind(fx.ourLongProperty)"/>
...

In OurBeanClass the ourLongProperty it's a Long. When we save our form if the intbox "ourintbox" is empty the binding mechanism saves in ourLongProperty a 0 value instead of null.