BigQuery Storage Read API source throws NullPointerException when a source table is not found

When Reading from BigQueryIO using DIRECT_READ getting 
```

~ Channel ManagedChannelImpl{logId=1, target=bigquerystorage.googleapis.com:443} was not shutdown properly!!!
~ Make sure to call shutdown()/shutdownNow() and wait until awaitTermination() returns true. 
```

```

2021-05-12 13:25:49.319 CEST Error message from worker:
java.lang.NullPointerException
org.apache.beam.sdk.io.gcp.bigquery.BigQueryStorageSourceBase.split(BigQueryStorageSourceBase.java:105)
org.apache.beam.sdk.io.gcp.bigquery.BigQueryStorageTableSource.split(BigQueryStorageTableSource.java:40)
org.apache.beam.runners.dataflow.worker.WorkerCustomSources.splitAndValidate(WorkerCustomSources.java:294)
org.apache.beam.runners.dataflow.worker.WorkerCustomSources.performSplitTyped(WorkerCustomSources.java:216)
org.apache.beam.runners.dataflow.worker.WorkerCustomSources.performSplitWithApiLimit(WorkerCustomSources.java:200)
org.apache.beam.runners.dataflow.worker.WorkerCustomSources.performSplit(WorkerCustomSources.java:179)
org.apache.beam.runners.dataflow.worker.WorkerCustomSourceOperationExecutor.execute(WorkerCustomSourceOperationExecutor.java:82)
org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.executeWork(BatchDataflowWorker.java:420)
org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.doWork(BatchDataflowWorker.java:389)
org.apache.beam.runners.dataflow.worker.BatchDataflowWorker.getAndPerformWork(BatchDataflowWorker.java:314)
org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.doWork(DataflowBatchWorkerHarness.java:140)
org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:120)
org.apache.beam.runners.dataflow.worker.DataflowBatchWorkerHarness$WorkerThread.call(DataflowBatchWorkerHarness.java:107)
java.util.concurrent.FutureTask.run(FutureTask.java:266)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
java.lang.Thread.run(Thread.java:748)

```

Sbt/Scala minimal example to reproduce [https://github.com/fpopic/BEAM-12359](https://github.com/fpopic/BEAM-12359) (if required can try to make it in Java).

Relates to user mailing list [question](https://lists.apache.org/x/thread.html/r705ca8f0ea9517f148637be59e509ce80fa75b22c98ad63acd551065@%3Cuser.beam.apache.org%3E).

Imported from Jira [BEAM-12359](https://issues.apache.org/jira/browse/BEAM-12359). Original Jira may contain additional context.
Reported by: fpopic.