fix NPE occuring at works termination during shutdown

When shut-downing the works manager, we've got NPEs on running work threads.

2014-08-30 18:50:40,774 ERROR Nuxeo-Work-default-2 [ecm.core.work.WorkManagerImpl] Uncaught error on thread Nuxeo-Work-default-2
java.lang.NullPointerException
at org.nuxeo.ecm.core.work.redis.RedisWorkQueuing.closeJedis(RedisWorkQueuing.java:367)
at org.nuxeo.ecm.core.work.redis.RedisWorkQueuing.getScheduledQueueSize(RedisWorkQueuing.java:447)
at org.nuxeo.ecm.core.work.redis.RedisWorkQueuing.getScheduledSize(RedisWorkQueuing.java:260)
at org.nuxeo.ecm.core.work.redis.RedisBlockingQueue.size(RedisBlockingQueue.java:124)
at java.util.AbstractCollection.isEmpty(AbstractCollection.java:86)
at java.util.concurrent.ThreadPoolExecutor.tryTerminate(ThreadPoolExecutor.java:690)
at java.util.concurrent.ThreadPoolExecutor.processWorkerExit(ThreadPoolExecutor.java:1006)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1163)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:744)