[serve] Fix AttributeError when request_router is None in update_deployment_config (ray-project#63180)

## Description

`AsyncioRouter.update_deployment_config()` unconditionally evaluates
`len(self.request_router.curr_replicas)` at the end of the method.
The `request_router` property performs lazy initialisation and returns
`None` when `_request_router_class` is `None` and `_request_router`
has not yet been initialised. In that state the call raises:

```
AttributeError: 'NoneType' object has no attribute 'curr_replicas'
```

**Trigger conditions**

1. `AsyncioRouter` is constructed without a `request_router_class` (the
   parameter is `Optional`), and the Controller's first long-poll config
   push arrives before any replica is assigned — the lazy-init path is
   never entered, so the property returns `None`.
2. A live deployment is hot-updated to remove a custom
   `request_router_class`; `update_deployment_config()` overwrites
   `self._request_router_class` with the new value before the guard
   `if self._request_router:` is evaluated, leaving both fields `None`.
3. Race condition: the Controller pushes a new config between
construction
and the first call to `assign_request()` that would trigger lazy init.

**Fix**

Cache the property result in a local variable and fall back to `0` when
it is `None`. Zero is semantically correct: no replicas are active yet,
so `MetricsManager` should not trigger a scaled-to-zero optimised push.

```python
# before
curr_num_replicas=len(self.request_router.curr_replicas),

# after
_router = self.request_router
curr_num_replicas = len(_router.curr_replicas) if _router is not None else 0
```

## Related issues

None — this is a self-contained bug fix.

## Checks

- [x] I've signed off every commit with `Signed-off-by`
- [x] I've run `scripts/format.sh` to lint the changes in this PR
- [x] I've included any doc changes needed for this PR

Signed-off-by: chenshi5012 <chenshi5012@163.com>

Author: chenshi5012

python/ray/serve/_private/router.py

@@ -796,9 +796,15 @@ def update_deployment_config(self, deployment_config: DeploymentConfig):
                 max_backoff_s=self._max_backoff_s,
             )
 
+        # Guard against the case where request_router is None (e.g., when
+        # request_router_class is None and lazy initialization has not yet
+        # occurred). In that scenario there are no active replicas yet, so
+        # passing 0 is the correct and safe value.
+        _router = self.request_router
+        curr_num_replicas = len(_router.curr_replicas) if _router is not None else 0
         self._metrics_manager.update_deployment_config(
             deployment_config,
-            curr_num_replicas=len(self.request_router.curr_replicas),
+            curr_num_replicas=curr_num_replicas,
         )
 
     async def _resolve_request_arguments(