This is a very pedantic question, but
Is the unique morphism from the empty scheme $\emptyset = \operatorname{Spec}((0))$ to some other scheme $X$ smooth?
This is a very pedantic question, but
Is the unique morphism from the empty scheme $\emptyset = \operatorname{Spec}((0))$ to some other scheme $X$ smooth?
Yes. It doesn't matter which definition you choose, smoothness of $\emptyset \to X$ is satisfied for trivial reasons. More generally, any open immersion is smooth.