swim: speed-up empty payloads cluster bootstrap

One another place consuming most of the tests start up time is
useless dissemination of an empty payload, which can be skipped
in fact.

Consider a cluster of 300 nodes. Each one of them are
interconnected manually, and now a test wants to wait for a
stabilization, when there are no events. On such a cluster it
happens for ~200 round steps till there are no any single event.

This is not about big packets, or log() TTD. There may be a few
events, may be more, but when a test wants the cluster to be
clean, it needs to wait for all the events being done.

This patch abuses the fact, that empty payloads can be compared
for free, no any single memcmp. If both new and the old payload
are empty, then nothing to disseminate.

It could help in a real cluster too, if initially there are no
payloads.

Needed for #4253