Describe the bug
Nodes often crashing (panic) during first few moments of Raft, when using distinct priority values per node together with pre_vote = true.
The least-prioritized node never crashes.

When I looked into it, I found that it's because during the pre-voting, we reject pre-voting requests from lower-priority nodes:
https://github.com/tikv/raft-rs/blob/82d704cdc3d93258be1f45efd715b95930764d7f/src/raft.rs#L1467C65-L1467C98

We then try to send the rejection back to the proposing node, including our term as the message's term:
https://github.com/tikv/raft-rs/blob/82d704cdc3d93258be1f45efd715b95930764d7f/src/raft.rs#L1494C21-L1498C58

But during that send, term is checked to be != 0, resulting in a panic instead of sending the proposal rejection:
https://github.com/tikv/raft-rs/blob/82d704cdc3d93258be1f45efd715b95930764d7f/src/raft.rs#L617C9-L640C14

Expected behavior
PreVoting rejection and not panic().