subject: avoid sending an extra empty subject update

When setSubject is called too early we store it as pensing, but thanks to the
default parameter value, if undefined is passed to the function we'll store the
empty string.

This will trigger a needless update because undefined !== ''.

react/features/base/conference/actions.js

@@ -763,12 +763,12 @@ export function setStartMutedPolicy(
  * @param {string} subject - The new subject.
  * @returns {void}
  */
-export function setSubject(subject: string = '') {
+export function setSubject(subject: string) {
     return (dispatch: Dispatch<any>, getState: Function) => {
         const { conference } = getState()['features/base/conference'];
 
         if (conference) {
-            conference.setSubject(subject);
+            conference.setSubject(subject || '');
         } else {
             dispatch({
                 type: SET_PENDING_SUBJECT_CHANGE,