An empty clause is never satisfiable. The clause contains one literal. NIL represents False in this context . In the first two cases, NIL represents an empty list or clause. Only in the third case does NIL represent a truth value.
T is only valid as a literal.
NIL here represents the empty clause set, not a literal. It turns out that these methods will help us solve the CNF-SAT or Satisfiability problem. The easiest approach to solving Satisfiability is to try to reduce the CNF formula to a logically equivalent yet simple base case where we know whether the expression is satisfiable. I hope this clarifies any confusion.