With the Message Composer for text-only email, HTML part is checked even if the HTML version is empty. So if we would like to send text-only email, we should turn off the check box HTML version　to avoid send empty messages.

In the content builder, if either HTML / TEXT is empty, only one of them will be delivered instead of multipart.

the checkbox is not checked because empty email body part.

As with the content builder, please implement it so that the check-box is off if the HTML content is empty.