发生切换时 leader 在执行自己 apply 的值时也可能拿到空的 callback

仅记录供其它开发者参考，不一定需要修复。

发生顺序如下：
状态机 on_leader_start, term = t1
(中间经过一段时间正常运行)
写 apply, expected_term = t1, value = xxx，得到错误：leader stepped down
状态机 on_apply, index = x1, term = t1, value = xxx, done = nullptr （状态机记录了 on_leader_start 时的term，可以发现这种极端情况）
状态机 on_leader_stop
后续 apply 时均会报错 is not leader

结论：
leader状态机不能假设自己apply的值一定会携带自己传入的done，即使满足如下条件：
1. 在自己任期
2. term相符
3. 没有调用 on_leader_stop

Start with the state-machine entry points named in the report: on_leader_start, apply, on_apply, and on_leader_stop. Trace the term and callback state through the reported leader step-down sequence, then determine the expected behavior when on_apply receives done = nullptr; the issue is marked as a reference report rather than a defined fix.

Written by the indexing model from the issue text.