ox-texinfo: Insert blank line between sectioning command and contents

* lisp/ox-texinfo.el (org-texinfo-headline): Insert blank line between
  sectioning command and contents.

lisp/ox-texinfo.el

@@ -877,7 +877,8 @@ holding contextual information."
 	      (funcall (plist-get info :texinfo-format-headline-function)
 		       todo todo-type priority text tags))
 	     (contents
-	      (concat (if (org-string-nw-p contents)
+	      (concat "\n"
+		      (if (org-string-nw-p contents)
 			  (concat "\n" contents)
 			"")
 		      (let ((index (org-element-property :INDEX headline)))