[FIX] handling of groups with length 0 in ContainerDataGroup's group descriptor munger

For a group with a ${field}_count of 0, code would fallback on __count
which does not exist -> would try to set group.length to `undefined`
which leads to the property being removed from the result, blowing up
the call to `$('<td>').text(group.length)` in the listview (jQuery's
.text() apparently does not like getting `undefined` as its
parameter... who knew?)

Also make `openable` clearer, and improve style of listview in case of
an empty group (openable but length 0): remove the arrow on the left
(but leave the indent, otherwise it's ugly).

lp bug: https://launchpad.net/bugs/889807 fixed

bzr revid: xmo@openerp.com-20111115112906-e87sf40tgr8goqps