fix(serve): guard against None request_router in update_deployment_config

When AsyncioRouter.update_deployment_config() is called before the
request_router has been lazily initialised (e.g. request_router_class
is None, or the first config update arrives before any replica is
assigned), the self.request_router property returns None.

The previous code unconditionally evaluated:
    len(self.request_router.curr_replicas)
which raises AttributeError: 'NoneType' object has no attribute
'curr_replicas'.

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

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(