Running Producer with empty dataset fails

[mafrahm]

Running a producer on an (post-selection) empty dataset seems to not work anymore. I suspect this happened due to the PR where we changed the strategy of our ChunkedIOReader (#345)

iterate through 0 events in 0 chunks ...
failed (took 0.49 seconds)
ERROR: luigi-interface - [pid 530] Worker Worker(salt=6786987636, workers=1, host=naf-cms11.desy.de, username=frahmmat, pid=40574) failed    cf.ProduceColumns(effective_workflow=local, branch=0, analysis=hbw.analysis.hbw_sl.hbw_sl, version=prod1, config=c17, shift=nominal, local_shift=nominal, dataset=qcd_mu_pt15to20_pythia, calibrators=skip_jecunc, selector=sl, producer=ml_inputs)
Traceback (most recent call last):
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/luigi/worker.py", line 203, in run
    new_deps = self._run_get_new_deps()
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/luigi/worker.py", line 138, in _run_get_new_deps
    task_gen = self.task.run()
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/modules/law/law/decorator.py", line 244, in wrapper
    return decorator(fn, _opts, *args, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/modules/law/law/decorator.py", line 272, in log
    return fn(task, *args, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/modules/law/law/decorator.py", line 244, in wrapper
    return decorator(fn, _opts, *args, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/modules/law/law/decorator.py", line 540, in localize
    return fn(task, *args, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/modules/law/law/decorator.py", line 231, in wrapper
    result = call(state)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/modules/law/law/decorator.py", line 303, in call
    return fn(task, *args, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/tasks/production.py", line 111, in run
    for (events, *cols), pos in self.iter_chunked_io(
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/tasks/framework/mixins.py", line 1865, in iter_chunked_io
    for obj in self.iter_progress(handler, max(handler.n_chunks, 1), msg=msg):
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/modules/law/law/task/base.py", line 565, in iter_progress
    for i, val in enumerate(iterable):
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/columnar_util.py", line 3372, in __iter__
    yield from self._iter_impl()
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/columnar_util.py", line 3303, in _iter_impl
    result_obj = result.get()
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/conda/lib/python3.9/multiprocessing/pool.py", line 771, in get
    raise self._value
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/conda/lib/python3.9/multiprocessing/pool.py", line 125, in worker
    result = (True, func(*args, **kwds))
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/columnar_util.py", line 3267, in read
    chunks.append(read_func(chunk_pos))
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/columnar_util.py", line 3158, in read_awkward_parquet
    return source_object.materialize(
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/columnar_util.py", line 2510, in materialize
    return self._materialize_via_slices(*args, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/modules/columnflow/columnflow/columnar_util.py", line 2527, in _materialize_via_slices
    return self.dak_array[entry_start:entry_stop].compute()
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/dask/base.py", line 342, in compute
    (result,) = compute(self, traverse=False, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/dask/base.py", line 628, in compute
    results = schedule(dsk, keys, **kwargs)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/dask_awkward/lib/io/parquet.py", line 124, in __call__
    return ak.Array(unproject_layout(self.original_form, array.layout))
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/dask_awkward/lib/unproject_layout.py", line 411, in unproject_layout
    return _unproject_layout(form, layout, layout.length, layout.backend)
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/dask_awkward/lib/unproject_layout.py", line 269, in _unproject_layout
    _unproject_layout(
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/dask_awkward/lib/unproject_layout.py", line 257, in _unproject_layout
    content=_unproject_layout(
  File "/afs/desy.de/user/f/frahmmat/Projects/hh2bbww/data/software/venvs/venv_columnar_dev_3cbb5aff/lib/python3.9/site-packages/dask_awkward/lib/unproject_layout.py", line 377, in _unproject_layout
    raise AssertionError(f"unexpected combination: {type(form)} and {type(layout)}")
AssertionError: unexpected combination: <class 'awkward.forms.recordform.RecordForm'> and <class 'awkward.contents.bitmaskedarray.BitMaskedArray'>