ZooKeeperRunningJobsRegistry creates an empty znode

ZooKeeperRunningJobsRegistry#writeEnumToZooKeeper calls

this.client.newNamespaceAwareEnsurePath(zkPath).ensure(client.getZookeeperClient());

This creates an empty znode in zookeeper.  If the job manager is interrupted at this point the job manager cannot recover.  When trying to restore jobs on a restarted job manager, ZooKeeperRunningJobsRegistry#getJobSchedulingStatus will throw an exception due to the empty znode. 

Behavior was verified in a test environment where the job manager was interrupted at that point in execution leaving ZK in the following state:

zk: localhost:2181(CONNECTED) 2] ls /flink/default
[checkpoint-counter, checkpoints, jobgraphs, leader, leaderlatch, running_job_registry]
[zk: localhost:2181(CONNECTED) 3] ls /flink/default/running_job_registry 
[c982053dd0b9100967e6a9d89202f2a5]
[zk: localhost:2181(CONNECTED) 4] get /flink/default/running_job_registry/c982053dd0b9100967e6a9d89202f2a5 

[zk: localhost:2181(CONNECTED) 5]