@ChiaraAni Did you try it?
user.update!(password: nil) and user.update!(password: "") and user.update!(password: " ") will all fail the validation, but user.update(name: 'Joana') works fine because the password is not being updated. You do not need to allow nil. Maybe the misunderstanding here is that user.update(name: "Foo") and user.update(name: "Foo", password: nil) are different. There's a very important difference between passing in a key with a value of nil and not passing in a key at all.