Put simply it isn’t a joke. It’s evolved to the point where it basically means “prepare yourself, a joke’s coming.”
It’s just a meme that got so overused that it doesn’t mean anything anymore.
Think of it like how 90% of knock knock jokes don’t need the setup of answering the door, it’s just a familiar setup. Why is a banana knocking on the door? Why does there need to be a door in the setup of interrupting cow? That’s what “no one” means to younger people. It’s a familiar way to set up the joke.
Edit: I forgot to mention, correct usage would be something like:
Nobody:
Me: A trillion lions could totally defeat the sun.
The joke being nobody asked, nobody cares, and I said it anyway.