Handle new XAUTOCLAIM response introduced in redis>=4.3.4

[A breaking change](redis/redis-py#2252) in `redis==4.3.4` breaks Cog.

This PR handles the new XAUTOCLAIM response format in a backwards-compatible way.

Closes replicate#671

Author: andreasjansson

python/cog/server/redis_queue.py

@@ -124,7 +124,8 @@ def receive_message(self) -> Tuple[Optional[str], Optional[str]]:
             1,
         )
         # format: [[b'1619393873567-0', [b'mykey', b'myval']]]
-        if raw_messages and raw_messages[0] is not None:
+        # since redis==4.3.4 an empty response from xautoclaim is indicated by [[b'0-0', []]]
+        if raw_messages and raw_messages[0] is not None and len(raw_messages[0]) == 2:
             key, raw_message = raw_messages[0]
             assert raw_message[0] == b"value"
             return key.decode(), raw_message[1].decode()