Category heading php question

I’m referencing this page:
http://codex.wordpress.org/Template_Tags/wp_list_categories
to include categories in my sidebar. However, I want to delete the default heading/title of my category list (or set a custom word with an <H2> ).

The documentation says:
wp_list_categories()

The title_li parameter sets or hides a title or heading for the category list generated by wp_list_categories. It defaults to ‘(__(‘Categories’)’, i.e. it displays the word “Categories” as the list’s heading. If the parameter is set to a null or empty value, no heading is displayed. The following example code excludes categories with IDs 4 and 7 and hides the list heading:

<ul>
<?php
wp_list_categories('exclude=4,7&amp;title_li='); ?>
</ul>

I just don’t know how to set the parameter to null/empty value. ???