Not sending an msg.payload in function node

I would know if it's possible to send each 90 pulses a msg.payload. So the other 89 pulses no msg.payload to the next node

Yes look at the doc's

I don't understand how it works

A message is sent from a function ONLY when return msg is reached OR node.send(msg) is called.

Therefore, if you move return msg to inside your if( I >= 90 ) { ... } block, that will make sending messages conditional.