Piecemeal conversion to draft needs to allow

already converted children.
STUD-1965

@cahrens please review (feel free to merge while I'm gone if it's ok)
I am a little confused why you are looking specifically for the published version. Yes, the published version should exist, but so should a draft version (since deleting private_leaf will make a draft version of the published parent vertical). Shouldn't you check that both the draft and published exist?