Python direct runner doesn't emit empty pane when it should

In test_stream_test.py, the test test_gbk_execution_no_triggers there is positive allowed_lateness. So the two windows [15, 30) and [300, 315) that do not have late data should have an empty pane emitted when window finishes (after allowed lateness) since the default ClosingBehavior in Python is EMIT_ALWAYS (see transforms/core.py).