See D20650. Currently, Ferret will not return a result if the document doesn't have a title, even if the document matches the search query in other fields (body, etc).
Empty fields are discarded by `PhabricatorFerretFulltextEngineExtension` and not inserted into the `ffield` table, but `PhabricatorCursorPagedPolicyAwareQuery` later does a `JOIN` against the `titl` field in this table to apply a ranking order.
I think we can fix this by turning the `JOIN` into a `LEFT JOIN` and maybe tweaking the ranking logic slightly.