fix: pytest raft_status assertion message

Since commit d87dd4ca `leader_id` became an `Option`, so the `None`
value isn't rendered in the `picolib.raft_status` response:

```python
status={'is_ready': False, 'raft_state': 'Follower', 'id': 1}
```

It makes pytest complain about missing argument:

```
cluster2 = Cluster("127.0.0.1:3300", n=2)

    def test_restart_leader(cluster2: Cluster):
        i1, i2 = cluster2.instances
        i1.assert_raft_status('Leader')
        i2.assert_raft_status('Follower')

        i1.restart()
>       i1.wait_ready()

test/int/test_joining.py:209:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../../.local/share/virtualenvs/picodata-6sv6l6y-/lib/python3.10/site-packages/funcy/decorators.py:45:
in wrapper
    return deco(call, *dargs, **dkwargs)
../../.local/share/virtualenvs/picodata-6sv6l6y-/lib/python3.10/site-packages/funcy/flow.py:127:
in retry
    return call()
../../.local/share/virtualenvs/picodata-6sv6l6y-/lib/python3.10/site-packages/funcy/decorators.py:66:
in __call__
    return self._func(*self._args, **self._kwargs)
test/int/conftest.py:305: in wait_ready
    status = self._raft_status()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = Instance(i1, listen=127.0.0.1:3301)

    def _raft_status(self) -> RaftStatus:
        status = self.call("picolib.raft_status")
        assert isinstance(status, dict)
        eprint(f"{status=}")
>       return RaftStatus(**status)
E       TypeError: RaftStatus.__init__() missing 1 required positional
argument: 'leader_id'

test/int/conftest.py:280: TypeError
```

This patch fixes the failure message:

```
self = Instance(i1, listen=127.0.0.1:3301)

    @funcy.retry(tries=20, timeout=0.1)
    def wait_ready(self):
        status = self._raft_status()
>       assert status.is_ready
E       AssertionError: assert False
E        +  where False = RaftStatus(id=1, raft_state='Follower',
is_ready=False, leader_id=None).is_ready

test/int/conftest.py:306: AssertionError
```