DimensionLookup: do not create unknown row when dimension table is empty

Any time the dimension table is empty, the DimensionLookup transform creates a new line with ID=0 that is treated as a garbage row. Apart from managing SCD dimension tables, this versioning logic could be interesting in other situations related to the transactional works (3rd normal form) where where that rows is not needed. In that case, we would like to have a flag that tells Hop not to create that line if the table is empty defined as false by default.