Show parent category if there is no subcategory

by

Your question is not that clear, I suggest you to improve your question.

You can try adding array parameter hide_empty to false in this case "0" so that it shows all the category even the one with no post.

$thisCat = get_categories(array('child_of'=>$_GET['cat'], 'hide_empty'=>0));

You can also refer the WordPress get_categories Codex

Hope this helps!