Whatsapp doesn't allow its users to post empty messages, or messages that only contain regular space characters.
To post an empty message, you can just use the invisible U+200E LEFT-TO-RIGHT MARK character! Click the "Copy" button below which copies the character into your clipboard, then paste into Twitter's tweet editor, and hit the "Send" button. Done!
This works both in Whatsapp's smartphone app and in the web client.