cli/edit: allow empty due dates in non-interactive mode

[fermino]

trafficstars

I ran into the necessity to batch-edit a bunch of tasks to unset the due date, and in doing so found that there's no supported way to "unset" it without using the interactive mode.

This PR aims to fix that. The current fix is somewhat quick-n-dirty, and I think it would be better to dynamically detect if the field is allowed to be None by using type annotations, but I'm just starting to dig into the codebase so I want to post it here first!

Anyways, thanks for all the work in the project :)

[WhyNotHugo]

Does this not unconditionally unset the due property is it's not specified in the command line?

E.g.: I specify --priority 3, and now my due date is removed.