v1.23.1

Fix: edit -D "" now clears the due date

edit <task> -D "" reported success but silently left the due date in place, despite --help promising an empty value clears it.

The Todoist API ignores an empty due_string (and rejects due: null); the only value it accepts as "remove the due date" is the natural-language string no date. The CLI was sending the empty string straight through, so the clear never happened. It now translates an empty --due into that clearing form, so the documented behaviour holds:

td edit "Pay rent" -D ""     # actually clears the due date now

This also brings --due in line with --deadline, whose empty-clears-it promise already worked.

Fixes #27.