empty comboitem renders without height

the attached example creates a combobox with a null value for unselecting a previously chosen value.

The problem is it is rendered with a narrow height (because it has no label or icon), so it hard to spot when using it. Apart from that the selection works as expected.

http://screencast.com/t/u8fnY655ijvq

A workaround is to set a minimum height for the comboitems:

<style>
	.z-comboitem {
		min-height: 20px;
	}
</style>