zephyr_mirror: Don't try subscribing to an empty stream list.

(imported from commit c8155459b768dd1fc192246c1f584abb30a7bc02)

api/bots/zephyr_mirror.py

@@ -134,6 +134,7 @@ def update_subscriptions_from_humbug():
         if stream in current_zephyr_subs:
             continue
         streams_to_subscribe.append((stream.encode("utf-8"), "*", "*"))
+    if len(streams_to_subscribe) > 0:
         zephyr_bulk_subscribe(streams_to_subscribe)
 
 def maybe_restart_mirroring_script():