show only sub categories if available?

use a conditional statement, for example:

$sub_cats = get_categories('parent=".get_query_var("cat'));
if( $sub_cats ) : 
//show list of child categories, for instance with wp_list_categories()
 echo '<ul>;
 wp_list_categories('title_li=&child_of=".get_query_var("cat'));
 echo '</ul>';
//or possibly using $sub_cats and a foreach loop//
 echo '<ul>';
 foreach( $sub_cats as $sub_cat ) {
   echo '<li><a href="'.get_category_link($sub_cat->term_id).'">'.$sub_cat->name.'</a></li>'; 
  }
 echo '</ul>';
else:
//the 'normal' LOOP of category.php//
endif;

http://codex.wordpress.org/Function_Reference/get_categories
http://codex.wordpress.org/Template_Tags/wp_list_categories

edit: list code for sub categories added