org-agenda.el (org-agenda-export-html-style): Default to nil

* org-agenda.el (org-agenda-export-html-style): Default to nil
as any string value will replace the htmlize style.

See commit 8447ea.

lisp/org-agenda.el

@@ -188,7 +188,7 @@ When this variable nil, the URL will (also) be shown."
   :group 'org-agenda
   :type 'boolean)
 
-(defcustom org-agenda-export-html-style ""
+(defcustom org-agenda-export-html-style nil
   "The style specification for exported HTML Agenda files.
 If this variable contains a string, it will replace the default <style>
 section as produced by `htmlize'.