Fix void-variable global-whitespace-mode

In case whitespace-mode hasn't been loaded yet.

core/core-ui.el

@@ -142,7 +142,7 @@ Does nothing if `whitespace-mode' or 'global-whitespace-mode' is already
 active or if the current buffer is read-only or not file-visiting."
   (unless (or (eq major-mode 'fundamental-mode)
               buffer-read-only
-              global-whitespace-mode
+              (bound-and-true-p global-whitespace-mode)
               (null buffer-file-name))
     (require 'whitespace)
     (set (make-local-variable 'whitespace-style)