As Wikipedia puts it,
... the only propositional connectives a formula in CNF can contain are and, or, and not. The not operator can only be used as part of a literal, which means that it can only precede a propositional variable or a predicate symbol.
But first-order logic also includes constant symbols true and false. Is it accurate to say that, as these are not variables, they must be eliminated before a formula can be considered valid CNF?
A clause is a disjunction of literals.
Thus, if we have $\bot$ in it, we can apply the equivalence $l \lor \bot \equiv l$ to cancel $\bot$.
But we you can also retain it; in conclusion we are trying to reach the "empty clause" that is exactly $\bot$.
If we end with two clauses $\lnot l$ and $l \lor \bot$ the resolution procedure will produce $\bot$, i.e. exactly $\square$, showing that the set of clauses is unsatisfiable.