Query for user reference field Drupal 8
$query = \Drupal::database()->select('node', 'n');
$query->fields('n', ['nid','type']);
$query->condition('type', array('basic'), 'IN'); // Basic is content type
$query->leftJoin('node__field_user_ref', 'UID', "n.nid = UID.entity_id"); // field_user_ref is entity field for user reference
$query->condition('UID.field_user_ref_target_id', NULL, 'IS NOT NULL');
$pager = $query->extend('Drupal\Core\Database\Query\PagerSelectExtender')->limit(50);
$results = $pager->execute()->fetchAll();