Backup fails for subcollections unless document exists for parent document

[Venryx]

See details here: willhlaw#40 (comment)

It's the same issue with both this library and the derivative one above. Basically, subcollections are ignored if there is not a document at the container path. (so subcollection items at "versions/v1/tasks/..." aren't backed up unless a "placeholder" document is created at "versions/v1")