list order by is breaking if order by is "unset"

This is fixing the issue in case of non-ajaxfied list.
Not sure if it will cover all cases.

$dir = '-'; (set in list.js)
but $orderbyRaw still contains the last orderby element name (so if the sanitizing is skipped for '-' it's doing a " ...order by element - " and so breaking again.

See https://github.com/Fabrik/fabrik/issues/1288