Yes, in Common Lisp, NIL is a value (it's a symbol, and by convention also the empty list). But when used as a type specifier, NIL denotes the empty set. So no

Yes, in Common Lisp, NIL is a value (it's a symbol, and by convention also the empty list). But when used as a type specifier, NIL denotes the empty set. So no Lisp object is of that type, and an array with that element type cannot store any object.