Fix error when rendering limited account in web UI (#19413)

app/javascript/mastodon/components/avatar.js

@@ -64,7 +64,7 @@ export default class Avatar extends React.PureComponent {
         onMouseLeave={this.handleMouseLeave}
         style={style}
         role='img'
-        aria-label={account.get('acct')}
+        aria-label={account?.get('acct')}
       />
     );
   }