When having a zul with:

<?taglib uri="http://www.zkoss.org/dsp/web/core" prefix="c"?>
<label value="@load(c:formatNumber(item.price, '0.00'))" />

and item.price is a null value then zk gives a message: Cannot format given Object as a Number.

According to
https://www.zkoss.org/wiki/ZUML%20Reference/EL%20Expressions/Core%20Methods/formatNumber
it should be the same as CommonFns.formatNumber(Object, String)
which according to the documentation should return an empty string if a null value is passed to it.