Usually an _empty_ object simply doesn't contain any of whatever its highest-level member happens to be. You can talk about forests in terms of nodes and edges, but the conceptual vantage point of a forest is that it contains trees. If it's empty, it just **doesn't contain any trees**.
By extension then, it wouldn't contain any nodes or edges since a forest only contains nodes or edges if its corresponding trees contain those same nodes or edges.
**Extras:** I'm not sure of the author's intention here (whether a forest is defined to be an object containing trees or whether a forest is a graph whose connected components are trees), but in the first interpretation you can run into the interesting state of affairs where a forest has no nodes or edges but is still not empty. How you ask? Simple -- the forest _contains_ any positive number of _empty_ trees.