ox-html: Do not output "nil" for empty sections

* lisp/ox-html.el (org-html-section): Do not output "nil" when section
  has no contents.

Reported-by: T.F. Torrey <tftorrey@tftorrey.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/94464>

lisp/ox-html.el

@@ -3078,7 +3078,7 @@ holding contextual information."
 		(or (org-element-property :CUSTOM_ID parent)
 		    section-number
 		    (org-export-get-headline-id parent info))
-		contents)))))
+		(or contents ""))))))
 
 ;;;; Radio Target