It doesn't make sense to create an empty CDC generation because it does
not make sense to have a cluster with no tokens. Add a sanity check to
cdc::make_new_generation_description which fails if somebody attempts to
do that (i.e. when the set of current tokens + optionally bootstrapping
node's tokens is empty).
The function does not work correctly if it is misused, as we saw in
scylladb/scylladb#23897. While the function should not be misused in the
first place, it's better to throw an exception rather than crash -
especially that this crash could happen on the topology coordinator.
66acaa1bf8