This happens because the converter generates a document with only an alienMeta (the magic word) and an empty internal list. The logic for generating an empty paragraph to prevent an empty document doesn't kick in if the document is non-empty from the converter's perspective but only contains meta items.

Change 73745 had a related patch set uploaded by Esanders:
Extend empty document check to check for meta-only data

https://gerrit.wikimedia.org/r/73745

Change 73745 merged by jenkins-bot:
Extend empty document check to check for meta-only data

https://gerrit.wikimedia.org/r/73745

Fixed in the above commit, which we will push out today.