Fixed #8583: Modified the migration SQL described in the comments upg…

…rade docs to account for a problem found with freecomments using Postgres. Thanks to rajeshd for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8653 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/contrib/comments/upgrade.txt

@@ -62,10 +62,10 @@ run the following SQL:
     UPDATE django_comments SET user_name = (
         SELECT username FROM auth_user 
         WHERE django_comments.user_id = auth_user.id
-    );
+    ) WHERE django_comments.user_id is not NULL;
     UPDATE django_comments SET user_email = (
         SELECT email FROM auth_user 
         WHERE django_comments.user_id = auth_user.id
-    );
+    ) WHERE django_comments.user_id is not NULL;
 
     COMMIT;