only prefill-route if there is at least one prefill router

Signed-off-by: PeaBrane <yanrpei@gmail.com>

Author: PeaBrane

components/backends/vllm/src/dynamo/vllm/handlers.py

@@ -166,7 +166,10 @@ async def generate(self, request, context):
 
             try:
                 prefill_worker_id = None
-                if self.prefill_router_client is not None:
+                if (
+                    self.prefill_router_client is not None
+                    and self.prefill_router_client.instance_ids()
+                ):
                     # Use the prefill router to get best worker ID
                     best_worker_response = await anext(
                         await self.prefill_router_client.generate(