Python DirectRunner does not emit data at GC time

The following should succeed but does not:
```

test_options = PipelineOptions(flags=['--allow_unsafe_triggers'])
with TestPipeline(options=test_options)
as pipeline:
  pcoll = (
    pipeline
    | beam.Create([(1, 1), (1, 2), (1, 3), (1, 4)])
    |
WindowInto(
      window.GlobalWindows(),
      trigger=trigger.AfterCount(5),
      accumulation_mode=trigger.AccumulationMode.ACCUMULATING)
 
  | beam.GroupByKey())
  assert_that(pcoll, equal_to([(1, [1, 2, 3, 4])]))
```

However, it  currently fails, because  pcoll will be empty. It appears that the Direct Runner drops data if the trigger never fired.

Imported from Jira [BEAM-13078](https://issues.apache.org/jira/browse/BEAM-13078). Original Jira may contain additional context.
Reported by: zhoufek.