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

Author: jdeppe-pivotal

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

@@ -1256,7 +1256,7 @@ private static ZParams toZParams(Aggregate aggregate, Weights weights) {
 	@SuppressWarnings("unchecked")
 	private static Tuple toTuple(List<?> bytes) {
 
-		if (bytes.isEmpty()) {
+		if (bytes == null || bytes.isEmpty()) {
 			return null;
 		}
 

src/test/java/org/springframework/data/redis/connection/jedis/JedisClusterConnectionTests.java

@@ -2277,6 +2277,9 @@ public void zPopMinShouldWorkCorrectly() {
 	@EnabledOnCommand("BZPOPMIN")
 	public void bzPopMinShouldWorkCorrectly() {
 
+		assertThat(clusterConnection.bZPopMin(KEY_1_BYTES, 1, TimeUnit.MILLISECONDS))
+				.isNull();
+
 		nativeConnection.zadd(KEY_1_BYTES, 10D, VALUE_1_BYTES);
 		nativeConnection.zadd(KEY_1_BYTES, 20D, VALUE_2_BYTES);
 		nativeConnection.zadd(KEY_1_BYTES, 30D, VALUE_3_BYTES);
@@ -2302,6 +2305,9 @@ public void zPopMaxShouldWorkCorrectly() {
 	@EnabledOnCommand("BZPOPMAX")
 	public void bzPopMaxShouldWorkCorrectly() {
 
+		assertThat(clusterConnection.bZPopMax(KEY_1_BYTES, 1, TimeUnit.MILLISECONDS))
+				.isNull();
+
 		nativeConnection.zadd(KEY_1_BYTES, 10D, VALUE_1_BYTES);
 		nativeConnection.zadd(KEY_1_BYTES, 20D, VALUE_2_BYTES);
 		nativeConnection.zadd(KEY_1_BYTES, 30D, VALUE_3_BYTES);