Fix potential NullPointerException when using Jedis Cluster BZPOP*

[jdeppe-pivotal]

• 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.

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[jdeppe-pivotal]

This is an example stacktrace:

util.TestException: Encountered exception while performing Redis operations
	at redis.SpringDataRedisClient.doRedisOperations(SpringDataRedisClient.java:144)
	at redis.SpringDataRedisClient.doOps(SpringDataRedisClient.java:123)
	at redis.SpringDataRedisClient.HydraTask_doOps(SpringDataRedisClient.java:118)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at hydra.MethExecutor.execute(MethExecutor.java:173)
	at hydra.MethExecutor.execute(MethExecutor.java:141)
	at hydra.TestTask.execute(TestTask.java:197)
	at hydra.RemoteTestModule$1.run(RemoteTestModule.java:222)
Caused by: org.springframework.data.redis.RedisSystemException: Unknown redis exception; nested exception is java.lang.NullPointerException
	at org.springframework.data.redis.FallbackExceptionTranslationStrategy.getFallback(FallbackExceptionTranslationStrategy.java:53)
	at org.springframework.data.redis.FallbackExceptionTranslationStrategy.translate(FallbackExceptionTranslationStrategy.java:43)
	at org.springframework.data.redis.connection.jedis.JedisClusterConnection.convertJedisAccessException(JedisClusterConnection.java:784)
	at org.springframework.data.redis.connection.jedis.JedisClusterZSetCommands.convertJedisAccessException(JedisClusterZSetCommands.java:1238)
	at org.springframework.data.redis.connection.jedis.JedisClusterZSetCommands.bZPopMin(JedisClusterZSetCommands.java:407)
	at org.springframework.data.redis.connection.DefaultedRedisConnection.bZPopMin(DefaultedRedisConnection.java:986)
	at org.springframework.data.redis.core.DefaultZSetOperations.lambda$popMin$37(DefaultZSetOperations.java:560)
	at org.springframework.data.redis.core.RedisTemplate.execute(RedisTemplate.java:223)
	at org.springframework.data.redis.core.RedisTemplate.execute(RedisTemplate.java:190)
	at org.springframework.data.redis.core.AbstractOperations.execute(AbstractOperations.java:97)
	at org.springframework.data.redis.core.DefaultZSetOperations.popMin(DefaultZSetOperations.java:560)
	at org.springframework.data.redis.core.ZSetOperations.popMax(ZSetOperations.java:528)
	at redis.SpringDataRedisOperations.bzpopmax(SpringDataRedisOperations.java:416)
	at redis.OperationType.bzpopmax(OperationType.java:478)
	at redis.OperationType.apply(OperationType.java:539)
	at redis.SpringDataRedisClient.doRedisOperations(SpringDataRedisClient.java:137)
	... 10 more
Caused by: java.lang.NullPointerException
	at org.springframework.data.redis.connection.jedis.JedisClusterZSetCommands.toTuple(JedisClusterZSetCommands.java:1259)
	at org.springframework.data.redis.connection.jedis.JedisClusterZSetCommands.bZPopMin(JedisClusterZSetCommands.java:405)
	... 21 more

[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.