feat: use default if DASHBOARD_URL is not set

Author: asadali145

src/learning-header/AuthenticatedUserDropdown.jsx

@@ -11,7 +11,7 @@ import messages from './messages';
 
 const AuthenticatedUserDropdown = ({ intl, username, name }) => {
   const dashboardMenuItem = (
-    <Dropdown.Item href={getConfig().DASHBOARD_URL}>
+    <Dropdown.Item href={getConfig().DASHBOARD_URL ? getConfig().DASHBOARD_URL : `${getConfig().LMS_BASE_URL}/dashboard`}>
       {intl.formatMessage(messages.dashboard)}
     </Dropdown.Item>
   );