add_filter('get_comment_author', 'my_comment_author', 10, 1);

function my_comment_author( $author = '' ) {
// Get the comment ID from WP_Query

$comment = get_comment( $comment_ID );

if ( empty($comment->comment_author) ) {
if (!empty($comment->user_id)){
$user=get_userdata($comment->user_id);
$author=$user->first_name.' '.substr($user->last_name,0,1).'.'; // this is the actual line you want to change
} else {
$author = __('Ziyaretçi');
}
} else {
$author = $comment->comment_author;
}

return $author;
}