Fix null ref source in shard agent

[Hawxy]

I see the below exception pop up in our logs from time-to-time after a successful rebuild, after the shard has already stopped.

System.NullReferenceException: Object reference not set to an instance of an object.
   at Marten.Events.Daemon.ShardAgent.ExecuteBatch(ProjectionUpdateBatch batch)
   at Marten.Events.Daemon.ShardAgent.<>c__DisplayClass44_0.<<processRange>b__2>d.MoveNext()
--- End of stack trace from previous location ---
   at Marten.Events.Daemon.ProjectionDaemon.TryAction(ActionParameters parameters)

I assume it's either the tracker or command block already being set to null by this point.