Despite MongoDB having never used the extractor config, a case has come up in the wild where it's appeared in a config string, albeit with an empty value.

This happened because of an historical tool used for backups, which would explicitly set some configuration strings. It would set all strings that could have a persistent value, with an "empty string" fallback. As a result, some clusters that have been migrated from ancient Mongo versions have this key set with an empty value.

For this ticket, it's sufficient to check that the string is empty, otherwise fail with a reasonably obvious error message.