Only write id locations if any are defined

This is to avoid an empty file when no id's have been used at all.

lisp/ChangeLog

@@ -1,5 +1,8 @@
 2009-12-10  Carsten Dominik  <carsten.dominik@gmail.com>
 
+	* org-id.el (org-id-locations-save): Only write the id locations
+	if any are defined.
+
 	* org-archive.el (org-archive-all-done): Make this work in a file
 	with org-odd-levels-only set.
 

lisp/org-id.el

@@ -466,7 +466,7 @@ When CHECK is given, prepare detailed information about duplicate IDs."
 
 (defun org-id-locations-save ()
   "Save `org-id-locations' in `org-id-locations-file'."
-  (when org-id-track-globally
+  (when (and org-id-track-globally org-id-locations)
     (let ((out (if (hash-table-p org-id-locations)
 		   (org-id-hash-to-alist org-id-locations)
 		 org-id-locations)))