Always add a deduplication entry to the metadata archive-file if deduplication is enabled

At the last workshop with BNF, it turned out that they expected to find
a deduplication-record in the metadata arc/warc file, when deduplication is enabled system-wide.

Until now, the record was not inserted unless the list of jobs used to generate the deduplication index was non-empty.

Now, a record will always be inserted if deduplication is on.