[FIX] trackers: Importing ItemLink fields with `lookup-simple` format fails if...

• [FIX] trackers: Importing ItemLink fields with lookup-simple format fails if the first 100 items in the linked field have empty values because enableLookup is always false, so by filtering them out the map returned always has meaningful values (thanks @jyhem and @robertokir - should probably replace 7e54e6e8 which was a bad fix by me a few years ago)