$drupal_hash_salt is empty in my installations
The hash is used to salt things one-time links and forms tokens, it filed usually by the installer.

You can add your own salt here, basically any random string or even replace it with the content of a file (see the documentation in the default.settings.php).

If this variable is empty, a hash of the serialized database credentials will be used as a fallback salt (though you have to add it manually if you have a database cluster).

This is function used as a fall back if there is no salt drupal_hash_salt

function drupal_get_hash_salt() {
  global $drupal_hash_salt, $databases;
  // If the $drupal_hash_salt variable is empty, a hash of the serialized
  // database credentials is used as a fallback salt.
  return empty($drupal_hash_salt) ? hash('sha256', serialize($databases)) : $drupal_hash_salt;
}