Term in raft can never be 0. It starts from 1 and can only grow.
It was assumed it can't be received from other nodes because they
do the same. There was an assertion for that.

But in raft_msg, used as a transport unit between raft nodes, it
was still possible to send 0 term. It could happen as a result of
a bug, or if someone would try to mimic the protocol but made a
mistake.

That led to a crash in the assert in debug build.

Part of #5303