Currently if you serialize an e-graph with an empty e-class (which was deleted) it is emitted as […] which is the same as if you set a max nodes cutoff and it was elided because of that.

We can't emit empty e-class in the serialized format, since nodes point directly to other nodes. So we do need to create a dummy variable. However, we could name that something else like "" so that it's clearer when visualizing.

First raised in #egg/egglog > "[...]" in the e-graph?