[BUGFIX][NIXL] quick fix for 'assert self.connector_worker is not None' in get_kv_connector_stats (#26851)

Signed-off-by: Chendi Xue <chendi.xue@intel.com>

Author: xuechendi

vllm/distributed/kv_transfer/kv_connector/v1/nixl_connector.py

@@ -241,7 +241,8 @@ def get_block_ids_with_load_errors(self) -> set[int]:
         return self.connector_worker.get_block_ids_with_load_errors()
 
     def get_kv_connector_stats(self) -> KVConnectorStats | None:
-        assert self.connector_worker is not None
+        if self.connector_worker is None:
+            return None
         return self.connector_worker.get_kv_connector_stats()
 
     @classmethod