Index: trac/ticket/model.py
===================================================================
--- trac/ticket/model.py        (revision 4489)
+++ trac/ticket/model.py        (working copy)
@@ -242,7 +242,11 @@
             else:
                 cursor.execute("UPDATE ticket SET %s=%%s WHERE id=%%s" % name,
                                (self[name], self.id))
-            cursor.execute("INSERT INTO ticket_change "
+
+            # Avoid writing out a 'change' if it went from None to ''
+            if self._old[name] or self[name]:
+                cursor.execute(
+                           "INSERT INTO ticket_change "
                            "(ticket,time,author,field,oldvalue,newvalue) "
                            "VALUES (%s, %s, %s, %s, %s, %s)",
                            (self.id, when_ts, author, name, self._old[name],