fix(Jetstream Pt): batch returned in prefill initialized to None

This is required when there are no more tokens generated after prefill.

text-generation-inference/server/text_generation_server/jetstream_pt_support/generator.py

@@ -476,6 +476,7 @@ def prefill(self, batch: Batch) -> Tuple[List[Generation], CachedBatch]:
                 self.slots.append(slot)
                 len_active_slots += 1
 
+        batch = None
         if len_active_slots > 0:
             # Whatever initial batch these requests came from, we always return all pending requests in a single batch
             request_ids = [slot.request_id for slot in self.slots if slot.state == Slot.State.READY]