Sorry, it's my mistake, I meant not <code> tag, but <pre> tag.

To be used like this:

System.out.println("<pre>" + resultTest + "</pre>");

What exactly are we wrapping in code tags? For example:

System.out.println(<code>resultTest</code>);
or
<code>System.out.println(resultTest);</code>

These issues with "null" showing up in the most unexpected places of a result are very annoying... Although I guess that is an issue with the CodeWars site as opposed to the kata.

If you need to know what input your code is given, you may print the input in your code. :)
As CodeWars "Terminal" where the code is run is HTML, I recommend wrapping the information being printed in <pre> tags.

And, by the way, I would say recursion is extreme overkill for this kata. :)