Refactor VM and operators

internal/object/null.go

@@ -18,6 +18,10 @@ func (n *Null) String() string {
 	return n.Inspect()
 }
 
+func (n *Null) LogicalNot() Object {
+	return &Boolean{!n.Bool()}
+}
+
 func (n *Null) Compare(other Object) int {
 	if _, ok := other.(*Null); ok {
 		return 0