Improve envelope greeting

It may be more readable if you put `alive_people[0].last_name.blank?` in the if condition instead of `last_name_guess` so it would be symmetrical with the check for the spouse last name being blank. Then it would read logically as "one or the other last name is blank or both are the same". Rather than having to remember back to the if above, but feel free to take another opinion on this and I don't think it's too big a deal.