avatar: make code a bit clearer

Uses nullish coalescing and optional chaining.

react/features/base/avatar/components/Avatar.js

  */
 export function _mapStateToProps(state: Object, ownProps: Props) {
     const { colorBase, displayName, participantId } = ownProps;
-    const _participant = participantId && getParticipantById(state, participantId);
-    const _initialsBase = (_participant && _participant.name) || displayName;
+    const _participant: ?Object = participantId && getParticipantById(state, participantId);
+    const _initialsBase = _participant?.name ?? displayName;
     const screenShares = state['features/video-layout'].screenShares || [];
 
-    let _loadableAvatarUrl = _participant && _participant.loadableAvatarUrl;
+    let _loadableAvatarUrl = _participant?.loadableAvatarUrl;
 
     if (participantId && screenShares.includes(participantId)) {
         _loadableAvatarUrl = 'icon://share-desktop';