How unrequire comment text in wordpress post

I solve it:

step 1 – insert a random number in text body, because WordPress prevent users that duplicate comments.

step 2- set display:none to textarea of comment field

Finally copy this code in functions.php:

add_filter( 'comment_form_field_comment', 'set_default_comment_text', 10, 1 );

function set_default_comment_text() {

    $post_id = get_the_title();

    global $current_user;

    get_currentuserinfo();

    $comment_rand = rand(10,100000);

    $comment_field = '<textarea style="display:none" id="comment" name="comment" cols="45" rows="8" maxlength="65525" readonly="readonly">'.$comment_rand.'</textarea>';

    return $comment_field;
}