Bug: --notes="" errors / impossible to set an empty reminder body #85

It's not possible to add (or edit) a reminder, where you set the reminder body to an empty string.

> reminders add "Tasks" --due-date="today" "foobar" --notes=""
Error: Missing value for '--notes <notes>'

While in the case of reminders add you could just leave out the --notes="", in the case of reminder edit, this makes it impossible to remove an existing reminder body. Also, this makes writing small scripts a bit annoying, since you need to always validate that the body is not empty.