In the usual framework of material set theory, every empty subset is identical to the empty set. For this reason, it is common to write simply instead of . Even from a structural perspective, this is an abuse of language that is unlikely to cause any confusion.
In the context of topology, we often speak of the empty subspace. In point-set topology, this is indeed an empty subset of the set of points, but in point-free topology, a space is not necessarily the empty space just because it has no points, and the empty subspace is similarly subtle.