Nova Menu CPT: fix notice when we have no taxonomies

`Notice: Undefined variable: term_id in ...wp-content\plugins\jetpack\modules\custom-post-types\nova.php on line 965`
I would write it this way so the function always returns something.
```if ( ! isset( $term_id ) ) {
	return false;
}
return get_term( $term_id, self::MENU_TAX );```