Blocking: Do not block transition to a nil TODO state

Blocking: Do not block transition to a nil TODO state

Samuel Wales writes:

> I find that changing a task from a todoish state to blank
> causes blocking to occur.  I normally want all todo state
> changes except those to doneish states to be non-blocked.
>
> I think the issue might be that blank is special-cased, as
> if it were a doneish state.  I would prefer blank to be a
> todoish state just like any other.