Removing issue parent

[torarnv]

--parent doesn't take "" or none.

Same goes for --cycle, --project, --due, and --assignee

--priority accepts none today. Are you okey with using the same sentinel for other things?

For --parent its safe, as no issue has the key none, but in theory --assignee none would break things for a real user named none. If that's a real concern, how about "" as "none"? Other alternatives?