Steps to Reproduce

Run fiddle:
http://zkfiddle.org/sample/2jf1inu/8-Another-new-ZK-fiddle
-on each listbox, click on the menupopup button on the top-right corner of the Listheader
-click group

Current Result

in the 1st listbox, one group is created without any label
in the 2nd listbox, 2 groups are created based on listcell labels

Expected Result

both listbox should find a label

Debug Info

caused by Listheader.group(boolean)
https://github.com/zkoss/zk/blob/master/zul/src/org/zkoss/zul/Listheader.java#L570-L580

if (cell.getLabel() != null) is always true due to LabelElement.getLabel() never returning null.
(LabelElement.setLabel will convert null to "" automatically

Root Cause

else statement can not be reached under normal workflow

Workaround

use listcell label="" to set a label value