[FIX] web_kanban: do no read something you already know

When the groups are loaded in the kanban view, a `read` is done on the
relation model to load extra fields.

This `read` was done regardless if there really were extra fields to
read.

This is a priori not a big issue, except if the user has no read
access to the relation model and can only perform a `name_get`.

addons/web_kanban/static/src/js/kanban_view.js

@@ -229,7 +229,7 @@ var KanbanView = View.extend({
 
             // fetch group data (display information)
             var group_ids = _.without(_.map(groups, function (elem) { return elem.attributes.value[0];}), undefined);
-            if (options.grouped_by_m2o && group_ids.length) {
+            if (options.grouped_by_m2o && group_ids.length && group_by_fields_to_read.length) {
                 return new data.DataSet(self, options.relation)
                     .read_ids(group_ids, _.union(['display_name'], group_by_fields_to_read))
                     .then(function(results) {