In CSSListMenuDisplayer when top level menu item has childer but none of them is allowed to be displayed
only ending </ul> tag is printed which leads to corrupted HTML output.

This is caused by the fact that when printing starting tag <ul> there is a check:
                if (hasViewableChildren(menu)) {
                    out.println("\t\t<ul>");
                }

but ending tag </ul> is not guarded:
            // close the </ul> for the top menu
            if (level == 0) {
                out.println("</ul>");
            }