Clarify logic in `nth_value` function

[alamb]

          took me a while to figure out what's happening here; maybe instead check slice.nulls() explicitly and return None if it doesn't exist (i.e. there's no nulls)?

also the pre-existing short-circuiting might be useful in cases where the nulls() index exists but valid_indices() truly is empty (i.e. the array is completely nulls)?

Originally posted by @Blizzara in #14042 (comment)

[alamb]

I think this is a good initial issue for someone

[zjregee]

take