b:selectMultiMenu doesn't cope with empty values

[stephanrauh]

Setting one of the items of <b:selectMultiMenu /> to an empty string causes the request to fail if the empty value is selected:

<b:selectMultiMenu  value="#{trafficLightBean.color}" >
    <f:selectItem itemLabel="no color" itemValue="" />
    <f:selectItem itemLabel="Green" itemValue="g"   />
    <f:selectItem itemLabel="Red" itemValue="r"  />
</b:selectMultiMenu>