org-agenda: Make timestamp ordering match docs

* lisp/org-agenda.el: When `org-agenda-sort-notime-is-late' is non-nil,
an absent timestamp is now (correctly) treated as maximal.

Bug reported by: Charles Tam <me@charlest.net> in
https://list.orgmode.org/CAKu+9YVNPSHG4HVtPFa2L2tP6yBdjJ38Teyr8OSDmCg0ZEOe9Q@mail.gmail.com

lisp/org-agenda.el

@@ -7288,7 +7288,7 @@ When TYPE is \"scheduled\", \"deadline\", \"timestamp\" or
 \"timestamp_ia\", compare within each of these type.  When TYPE
 is the empty string, compare all timestamps without respect of
 their type."
-  (let* ((def (and (not org-agenda-sort-notime-is-late) -1))
+  (let* ((def (if org-agenda-sort-notime-is-late 99999999 -1))
 	 (ta (or (and (string-match type (or (get-text-property 1 'type a) ""))
 		      (get-text-property 1 'ts-date a))
 		 def))