metadata indexer: investigate metadata entries with empty mappings

! In #1549 (closed), @zack wrote: This is now done, aside from a minor issue noted below:

softwareheritage-indexer=# select count(*) from revision_intrinsic_metadata where mappings = array[]::text[];
 count 
-------
   372

These metadata entries shouldn't have been generated in the first place, but they are real metadata, with no indication where they have been extracted from.

We should investigate why they are there.

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Migrated from T1691 (view on Phabricator)