I see this alert only when I've accidentally pressed enter or tapped the send button twice, and I have to take extra work to dismiss an alert about it. No other chat clients I know of have an alert like this, and I'd rather we didn't either.
All the chat clients I have used in the past few years do absolutely nothing when I try to send a blank message. There isn't even an error anywhere that shows up to tell me what I'm doing wrong. They just trust that I'll see the total lack of responsiveness and either immediately, or sooner or later, go "oh! I can't send blank messages. Of course, that makes so much sense."
When my message box is empty, could the site instead just consider the SEND button disabled, maybe visually disabled, and not let me send the message, and provide no error message at all that I'd have to worry about dismissing / eating up page space unnecessarily?