You will get the below error when logging in to phpMyAdmin without password even if your actual MySQL account itself is passwordless. This is because phpMyAdmin is normally configured to disable passwordless login due to security reasons.

To enable passwordless MySQL login in phpMyAdmin, you'll need to set AllowNoPassword option in phpMyAdmin's configuration to TRUE.

To enable the option, uncomment or add the following line;

$cfg['Servers'][$i]['AllowNoPassword'] = TRUE;

Depending on distributions, there could be multiple occurrences of that particular line. Ubuntu for example has the following 2 lines;

if (!empty($dbname)) {
    // other configuration options
    $cfg['Servers'][$i]['AllowNoPassword'] = TRUE;
    // it should be placed before the following line
    $i++;
}

// other configuration options
$cfg['Servers'][$i]['AllowNoPassword'] = TRUE;