handling nil fix

ml.cpp

@@ -197,7 +197,7 @@ bool MlValue::is_list() const {
 }
 
 bool MlValue::as_bool() const {
-	return type != NIL;
+	return !(type == NIL || (type == LIST && list.empty()));
 }
 
 long MlValue::as_int() const {
@@ -949,7 +949,7 @@ MlValue if_then_else(std::vector<MlValue> args, MlEnvironment &env) {
 	else if (args.size() == 3)
 		return args[2].eval(env);
 
-	return MlValue(0l);
+	return MlValue::nil();
 }
 
 // cond (SPECIAL FORM), in lisp it's macro, but we don't have support for macros yet