While I normally don't like required field, categories or items created in the gradebook should have some type of a name or title so that one can know what they are referring to after they have been created.
Add this line to the file /grade/edit/tree/category_form.php after $mform->addElement('text', 'fullname', get_string('categoryname', 'grades'));
$mform->addRule('fullname', null, 'required', null, 'client');
That was easy. A CVS patch is attached