The handling of children for non-existent nodes in ZooCache may not be correct.

[keith-turner]

Describe the bug

ZooCache attempts to cache nonexistence. This may not be done correctly for the case of getChildren.

The ZooCache code expects that ZooKeeper.getChildren() may return null here. However it will probably never return null. When a node in zookeeper exists and has no children an empty list is returned. When a node does not exist a NONODE KeeperException is thrown. The code zoocache code is handling this, however it does not cache this. The code behaves correctly, this is only a possible performance bug.

Expected behavior

• Remove handling for null in code as its not expected to ever happen.
• Determine if its safe to cache non-existence for getChildren, which depends on ZK watcher behavior. If it is safe then look into changing the code to cache this. If not then only the null handling needs to be removed for this issue.