Don't change notification sound to empty on mute.

If notifications are muted no one from official apps play sound.
So there is no need to turn the sound off, otherwise if you enable
the notifications from a different app the sound stays off.

Enable the sound (change to "default") when turning notifications on.

Telegram/SourceFiles/mainwidget.cpp

@@ -4016,7 +4016,9 @@ void MainWidget::updateNotifySetting(PeerData *peer, NotifySettingStatus notify,
 	}
 	if (peer->notify != EmptyNotifySettings && peer->notify != UnknownNotifySettings) {
 		if (notify != NotifySettingDontChange) {
-			peer->notify->sound = (notify == NotifySettingSetMuted) ? "" : "default";
+			if ((notify != NotifySettingSetMuted) && peer->notify->sound.isEmpty()) {
+				peer->notify->sound = qsl("default");
+			}
 			peer->notify->mute = (notify == NotifySettingSetMuted) ? (unixtime() + muteFor) : 0;
 		}
 		if (silent == SilentNotifiesSetSilent) {