org-element: Fix suprious "nil" in dynamic-block interpreter

* lisp/org-element.el (org-element-dynamic-block-interpreter): Ignore
  empty arguments instead of displaying "nil".

Reported-by: John Kitchin <jkitchin@andrew.cmu.edu>
<http://permalink.gmane.org/gmane.emacs.orgmode/113551>

lisp/org-element.el

@@ -849,7 +849,7 @@ CONTENTS is the contents of the element."
   (format "#+BEGIN: %s%s\n%s#+END:"
 	  (org-element-property :block-name dynamic-block)
 	  (let ((args (org-element-property :arguments dynamic-block)))
-	    (and args (concat " " args)))
+	    (if args (concat " " args) ""))
 	  contents))