Allow (:priority nil) to match entries without explicit priorities

I would like to group items without an assigned priority, but `:priority nil` doesn't seem to work select those items.

Would you please confirm whether `:priority nil` is a valid selector option?

I haven't tried yet, but I think the nil grouping can still be achieved using something like: `:not (:priority>= "F")` (#F is my lowest priority).

I'm just asking in case it is an error on my part.

Thanks!