Remove default value from openDisplayNamePrompt action

modules/UI/UI.js

  * Prompt user for nickname.
  */
 UI.promptDisplayName = () => {
-    APP.store.dispatch(openDisplayNamePrompt());
+    APP.store.dispatch(openDisplayNamePrompt(undefined));
 };
 
 /**

react/features/display-name/actions.js

  * submit of the dialog.
  * @returns {Object}
  */
-export function openDisplayNamePrompt(onPostSubmit: ?Function = undefined) {
+export function openDisplayNamePrompt(onPostSubmit: ?Function) {
     return openDialog(DisplayNamePrompt, {
         onPostSubmit
     });