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.