[FIX] web: display prev/next arrow after grouping

On a list view, if we group records the arrows and changing page
feature are disabled. But if then we removed the grouping, the arrows
never reappeared.

note: not necessary as of 9.0 it was already solved in 1280bf251

opw-760956
closes #18596

addons/web/static/src/js/view_list.js

@@ -1628,6 +1628,7 @@ instance.web.ListView.Groups = instance.web.Class.extend( /** @lends instance.we
                     self.render_groups(groups));
                 if (post_render) { post_render(); }
             }, function (dataset) {
+                self.view.$pager.find('.oe_page_group').css('display', '');
                 self.render_dataset(dataset).then(function (list) {
                     self.children[null] = list;
                     self.elements =