[FIX] website: allow publishers to save their website edition

On save, the editor will ask to parent widgets if something needs to be
done first. The website navbar will receive that demand and asks to its
action-capable components to do something. For example, the content menu
handles page-related options saving. However, some users with limited
access rights do not have the content menu... but the website navbar
expects that the save action is performed. So, this empty action is
defined here so that all users have an 'on_save' related action.

This was only happening since 12.0 as 'on_save' actions were introduced
there to save page-related options.

closes odoo/odoo#28810

addons/website/static/src/js/menu/edit.js

@@ -15,6 +15,7 @@ var EditPageMenu = websiteNavbarData.WebsiteNavbarActionWidget.extend({
     xmlDependencies: ['/website/static/src/xml/website.editor.xml'],
     actions: _.extend({}, websiteNavbarData.WebsiteNavbarActionWidget.prototype.actions, {
         edit: '_startEditMode',
+        on_save: '_onSave',
     }),
     custom_events: _.extend({}, websiteNavbarData.WebsiteNavbarActionWidget.custom_events || {}, {
         snippet_dropped: '_onSnippetDropped',
@@ -89,6 +90,19 @@ var EditPageMenu = websiteNavbarData.WebsiteNavbarActionWidget.extend({
             return def;
         });
     },
+    /**
+     * On save, the editor will ask to parent widgets if something needs to be
+     * done first. The website navbar will receive that demand and asks to its
+     * action-capable components to do something. For example, the content menu
+     * handles page-related options saving. However, some users with limited
+     * access rights do not have the content menu... but the website navbar
+     * expects that the save action is performed. So, this empty action is
+     * defined here so that all users have an 'on_save' related action.
+     *
+     * @private
+     * @todo improve the system to somehow declare required/optional actions
+     */
+    _onSave: function () {},
 
     //--------------------------------------------------------------------------
     // Handlers