Fix `org-skip-over-state-notes'

* lisp/org.el (org-skip-over-state-notes): Allow empty TODO states.

Reported-by: Michael Brand <michael.ch.brand@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2018-05/msg00337.html>

lisp/org.el

@@ -13594,8 +13594,8 @@ EXTRA is additional text that will be inserted into the notes buffer."
 		      (regexp-quote (cdr (assq 'state org-log-note-headings)))
 		      `(("%d" . ,org-ts-regexp-inactive)
 			("%D" . ,org-ts-regexp)
-			("%s" . "\"\\S-+\"")
-			("%S" . "\"\\S-+\"")
+			("%s" . "\\(?:\"\\S-+\"\\)?")
+			("%S" . "\\(?:\"\\S-+\"\\)?")
 			("%t" . ,org-ts-regexp-inactive)
 			("%T" . ,org-ts-regexp)
 			("%u" . ".*?")