Don't fail rejester work units that filter every document out.
Don't fail rejester work units that filter every document out.

That is, if you're running a filtering pipeline stage, and it causes
nothing to come out the far end of the pipeline, after not running any
writers, also don't try to update the work unit with its previous outputs
plus None (which will fail).

trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline