Use a different "unread messages" symbol if the messages were deleted

It's definitely a really annoying thing to check on a channel for unread messages only for it to be empty. This can happen as a result of bots/spam/etc, but it also puts someone in an awkward spot if they accidentally posted in the wrong channel.

Maybe it would be confusing if the unread marker could just disappear from a channel, which is why using a different marker would be less confusing.