ox-texinfo: Fix spurious "nil" in "special block export

* lisp/ox-texinfo.el (org-texinfo-special-block): Remove spurious
  "nil" when no option is defined.

lisp/ox-texinfo.el

@@ -1368,7 +1368,7 @@ as a communication channel."
 	(type (org-element-property :type special-block)))
     (format "@%s%s\n%s@end %s"
 	    type
-	    (if opt (concat " " opt) opt)
+	    (if opt (concat " " opt) "")
 	    (or contents "")
 	    type)))