You may cast Banishing Slash without choosing a target. If you do, and you control both an artifact and an enchantment as it resolves, you'll just create a Samurai token. However, if you do choose a target and that target is an illegal target as Banishing Slash tries to resolve, Banishing Slash won't resolve and none of its effects will happen. The illegal target won't be destroyed and you won't create a Samurai token.