GroupByKey doesn't seem to work with FixedWindows in DirectRunner

After applying `FixedWindows` on a streaming source, a `GroupByKey` operation won't emit keyed elements in a window. 

This example without `GroupByKey` prints all the windowed elements:

 
```

pipeline
.apply("ReadFromPubsub", PubsubIO.readStrings().fromSubscription(subscriptionPath))
.apply(Window.into(FixedWindows.of(Duration.standardSeconds(5L))))

.apply(WithKeys.of("bobcat"))
.apply(MapElements.into(TypeDescriptors.nulls()).via(
(KV pair) -> {
LOG.info("Key: " + pair.getKey() + "\tValue: " + pair.getValue());

return null;
}
));
```

This example with `GroupByKey` doesn't emit anything:

 
```

pipeline
.apply("ReadFromPubsub", PubsubIO.readStrings().fromSubscription(subscriptionPath))
.apply(Window.into(FixedWindows.of(Duration.standardSeconds(5L))))

.apply(WithKeys.of("bobcat"))
.apply(GroupByKey.create())
.apply(FlatMapElements.into(TypeDescriptors.nulls()).via(

(KV> pair) -> {
pair.getValue().forEach(message -> LOG.info("Message:
" + message));
return null;
}
));
```

I'm using DirectRunner. The same logic works for Python using both the DirectRunner and DataflowRunner.

Imported from Jira [BEAM-12075](https://issues.apache.org/jira/browse/BEAM-12075). Original Jira may contain additional context.
Reported by: tianzi.

Run the Java reproducer with DirectRunner, using PubsubIO, FixedWindows, WithKeys, GroupByKey.create(), and FlatMapElements. Trace the windowed grouped output and compare it with the working example without GroupByKey. Done means keyed elements are emitted from each fixed window as expected.

Written by the indexing model from the issue text.