org-agenda.el (org-cmp-ts): Avoid error when trying to string-match against nil

* org-agenda.el (org-cmp-ts): Avoid error when trying to
string-match against nil.

TINYCHANGE

lisp/org-agenda.el

@@ -6984,14 +6984,14 @@ The optional argument TYPE tells the agenda type."
 
 (defsubst org-cmp-ts (a b &optional type)
   "Compare the timestamps values of entries A and B.
-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."
+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 (if org-sort-agenda-notime-is-late 9901 -1))
-	 (ta (or (and (string-match type (get-text-property 1 'type a))
+	 (ta (or (and (string-match type (or (get-text-property 1 'type a) ""))
 		      (get-text-property 1 'ts-date a)) def))
-	 (tb (or (and (string-match type (get-text-property 1 'type b))
+	 (tb (or (and (string-match type (or (get-text-property 1 'type b) ""))
 		      (get-text-property 1 'ts-date b)) def)))
     (cond ((< ta tb) -1)
 	  ((< tb ta) +1))))