DirectRunner NullPointerException

When using the `DirectRunner` in tests I encountered flaky tests due to `NullPointerException`.

When the test fails the relevant stack trace is like below:

 
```

java.lang.NullPointerException
at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Preconditions.checkNotNull(Preconditions.java:877)

at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.util.concurrent.ThreadFactoryBuilder.setThreadFactory(ThreadFactoryBuilder.java:132)

at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:192)
at org.apache.beam.runners.direct.DirectRunner.run(DirectRunner.java:67)

at org.apache.beam.sdk.Pipeline.run(Pipeline.java:322)
at google.registry.beam.TestPipelineExtension.run(TestPipelineExtension.java:344)

at google.registry.beam.TestPipelineExtension.run(TestPipelineExtension.java:324)
at google.registry.beam.spec11.Spec11PipelineTest.testSuccess_saveToGcs(Spec11PipelineTest.java:185)
```

 
However the null pointer seems to have come from calling (a repackaged and very innocent looking) guava function to set the `ThreadFactory`:
 
```

 ExecutorService metricsPool =
Executors.newCachedThreadPool(
new ThreadFactoryBuilder()
.setThreadFactory(MoreExecutors.platformThreadFactory())

.setDaemon(false) // otherwise you say you want to leak, please don't!
.setNameFormat("direct-metrics-counter-committer")

.build());
```

 
Which in turn is just calling java.util.concurrent.Executors.defaulThreatFactory() when not on App Engine:
 
```

@Beta
@GwtIncompatible
public static ThreadFactory platformThreadFactory() {
if (!isAppEngine())
{
  return Executors.defaultThreadFactory();
  } else {
...
} 
```

I don't understand how this could possibly have failed. The test seems to be only flaky on our CI servers running Ubuntu and I haven't been able to reproduce it locally, making debugging a challenge.

I'd appreciate any input you may have. Thanks!

Imported from Jira [BEAM-12215](https://issues.apache.org/jira/browse/BEAM-12215). Original Jira may contain additional context.
Reported by: jianglai.