Handle broadcasts to zero clients

Fixes #88

socketio/asyncio_manager.py

@@ -23,7 +23,8 @@ class AsyncManager(BaseManager):
                     id = None
                 tasks.append(self.server._emit_internal(sid, event, data,
                                                         namespace, id))
-        await asyncio.wait(tasks)
+        if tasks != []:
+            await asyncio.wait(tasks)
 
     async def close_room(self, room, namespace):
         """Remove all participants from a room.