I've noticed that if you post a message in chat, press the "up arrow" or click "edit", and then press enter without changing anything (instead of pressing cancel), a little pencil will appear by the message indicating that the message was edited.
I decided to post a chat message, and then follow the steps above. I then went into the message history, and this is what I saw:
As you can see, there are two versions of the exact same thing. I have even seen messages that were edited multiple times, but had several versions of the exact same message.
I feel that it would be more intuitive for the edit to be canceled when nothing is changed, instead of appending a new copy of the same message to the history, and making the message appear as if it were edited. As some have stated, if you edit a normal Q/A post or comment, you can cancel the edit by pressing enter. So I think that it would only make sense to make the behavior be the same in chat.
Also, one idea for an implementation that I have, is to make the "cancel" button be invisible until you start typing. Of course this would mainly serve as a visual indication that pressing enter/send will have no effect. It may also be a good idea to put a small message that states that the message will not be changed.