ZkStateWriter#enqueueUpdate should execute callback even tho cmds is empty or no-op

ZkStateWriter#enqueueUpdate(ClusterState prevState, List<ZkWriteCommand> cmds, ZkWriteCallback callback) accepts a callback but such callback would not be called if either the cmds is empty or a no-op.

However, this could cause issues in some scenarios, for example in Overseer$ClusterStateUpdater#run, it calls

clusterState = processQueueItem(message, clusterState, zkStateWriter, true,
() ->

{ stateUpdateQueue.remove(processedNodes); processedNodes.clear(); }

);

Which it relies on the callback to remove items from the work queue, however if the message does not create an empty List<ZkWriteCommand> - for example "DOWNNODE" for PRS enabled collection, then the callback will be ignored hence the `stateUpdateQueue.remove(processedNodes);` would not be run even though it has finished processing the message