Exit hadoop map if only one filename is submitted

See #331

If only one filename is delivered the hadoop map process shouldn't start,
as it doesn't make sense to collect triple subjects without mapping their
triple objects to anything (well, it could be that this one file is a deep
graph in its own and that it should be processed so that resulting records
have raised triples clinging to the top subject URI (see e.g. hbz/lobid#28
"removing resolved values"), but for now we take it that this is not the
wanted default behaviour. If you need this, pass two identical filenames ).