org-html.el (org-html-expand): Return nil if STRING is nil.

* org-html.el (org-html-expand): Prevent a nil value for
STRING to return an error, just return nil.

lisp/org-html.el

@@ -2310,18 +2310,20 @@ Possible conversions are set in `org-export-html-protect-char-alist'."
 
 (defun org-html-expand (string)
   "Prepare STRING for HTML export.  Apply all active conversions.
-If there are links in the string, don't modify these."
-  (let* ((re (concat org-bracket-link-regexp "\\|"
-		     (org-re "[ \t]+\\(:[[:alnum:]_@#%:]+:\\)[ \t]*$")))
-	 m s l res)
-    (while (setq m (string-match re string))
-      (setq s (substring string 0 m)
-	    l (match-string 0 string)
-	    string (substring string (match-end 0)))
-      (push (org-html-do-expand s) res)
+If there are links in the string, don't modify these.  If STRING
+is nil, return nil."
+  (when string
+    (let* ((re (concat org-bracket-link-regexp "\\|"
+		       (org-re "[ \t]+\\(:[[:alnum:]_@#%:]+:\\)[ \t]*$")))
+	   m s l res)
+      (while (setq m (string-match re string))
+	(setq s (substring string 0 m)
+	      l (match-string 0 string)
+	      string (substring string (match-end 0)))
+	(push (org-html-do-expand s) res)
       (push l res))
-    (push (org-html-do-expand string) res)
-    (apply 'concat (nreverse res))))
+      (push (org-html-do-expand string) res)
+      (apply 'concat (nreverse res)))))
 
 (defun org-html-do-expand (s)
   "Apply all active conversions to translate special ASCII to HTML."