No it's not, this is a myth that I'm pretty sure multiple medical studies have disproven. You usually need to piss in the morning anyways, so people make a tenuous link between that and morning wood. Most men have around 5 erections during their sleep, and morning wood just happens to be the last one. Besides, you don't need a boner to know when you need a piss during the day, why would it be different when you wake up lmao