Don't try to get the value of unbound symbols

With `doxymacs-mode` installed the error message `Error during redisplay: (eval (sml/generate-minor-modes)) signaled (void-variable doxymacs-mode)` is continuously printed to the message buffer until `doxymacs.el` is loaded since `doxymacs-mode` is registered as a minor mode by autoload but doesn't have a value yet.