Pipeline does not handle "no document" case properly

The incremental ingester and its pipeline code, and indeed transformation connectors themselves, do not handle the "no document" case properly.

What should happen is that whenever a document is not retransmitted into a branch of the pipeline, the document version should nevertheless be recorded in the ingeststatus table for the corresponding output. But transformation connectors can interfere with that transmission if they decide simply to not index an incoming document. They may also receive a null RepositoryDocument into the transformation method, and will be unequipped to handle that.

Instead, I propose the following:
(1) An explicit IIncrementalIngester method needs to be invented for handling this case
(2) The activity class used for transformation methods should have a "noDocument()" method
(3) IncrementalIngester needs to propagate "noDocument" method calls down the pipeline without calling transformations on them