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 autoloaded but won't
have a value yet.