Table Transformer Callers should Expect null return value

[nbauernfeind]

DHE's transformer returns null on authorization issues. Maybe better would be to throw an Unauthorized style exception. These sources should fail.. but at the original time of implementation we could not throw when the transformer was invoked. We might need to revoke that decision and allow authorization style exceptions to be thrown. DHE does not want to return an empty table, which is what I had in mind the first time.

13:13:40.893 ERROR heduler-Concurrent-4 | i.d.s.s.SessionState      | Internal Error 'baed05b8-8ef3-4efd-87d9-2281ae3dcd27' java.lang.NullPointerException: Cannot invoke "io.deephaven.engine.table.Table.size()" because "table" is null
	at io.deephaven.extensions.barrage.util.ExportUtil.buildTableCreationResponse(ExportUtil.java:22)
	at io.deephaven.extensions.barrage.util.ExportUtil.buildTableCreationResponse(ExportUtil.java:14)
	at io.deephaven.server.partitionedtable.PartitionedTableServiceGrpcImpl.lambda$getTable$6(PartitionedTableServiceGrpcImpl.java:189)
	at io.deephaven.server.session.SessionState$ExportObject.doExport(SessionState.java:995)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at io.deephaven.server.runner.scheduler.SchedulerModule$ThreadFactory.lambda$newThread$0(SchedulerModule.java:100)
	at java.base/java.lang.Thread.run(Thread.java:840)