Fix undefined index in $var['body_classes']

I get a notice if there's no 'body_classes' in $var (it can easily happen if the subtheme is careless):

Notice: Undefined index: body_classes in zen_ninesixty_preprocess_page() (line 111 of /home/coornail/localhost/htdocs/sep/sites/all/themes/zen_ninesixty/template.php).