Fix potential NullPointerException when using Jedis Cluster BZPOP*.

For the Jedis-based implementations of bzpopmin and bzpopmax, a NPE is
  thrown if the provided timeout elapses and the server responds with a
  null array.

Closes #2324

src/main/java/org/springframework/data/redis/connection/jedis/JedisClusterZSetCommands.java

@@ -1044,7 +1044,7 @@ class JedisClusterZSetCommands implements RedisZSetCommands {
 	@SuppressWarnings("unchecked")
 	private static Tuple toTuple(List<?> bytes) {
 
-		if (bytes.isEmpty()) {
+		if (bytes == null || bytes.isEmpty()) {
 			return null;
 		}