Knock knock. Who's there? A rhetorical question.

A rhetorical question who? Don't answer that, it's rhetorical.

Knock knock. Who's there? A placeholder for a name.

A placeholder for a name who? Exactly. It could be anyone, couldn't it?

Knock knock. Who's there? The premise.

The premise who? The premise for a knock-knock joke needs two people, and I'm talking to myself.

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not sure, but the meaninglessness of this joke is a reflection of the void that lies at the heart of our existence.