Before starting this, you need to open web.whatsapp.com in your computer and sync your account with your mobile. Then go to the following website.
Here click on either zero width joiner or zero width non joiner. Choose copy and paste in the web.whatsapp.com under any chat to whom you wish to send the empty message. In ubuntu or linux we can type this character by holding Ctrl+Shift+U and press 200C
Here is how it looks.