BooleanOperatorString.php

<?php

namespace Drupal\views\Plugin\views\filter;

use Drupal\views\Attribute\ViewsFilter;

/**
 * Simple filter to handle matching of boolean values.
 *
 * This handler checks to see if a string field is empty (equal to '') or not.
 * It is otherwise identical to the parent operator.
 *
 * Definition items:
 * - label: (REQUIRED) The label for the checkbox.
 *
 * @ingroup views_filter_handlers
 */
class BooleanOperatorString extends BooleanOperator {
    public function query() {
        $this->ensureMyTable();
        $where = "{$this->tableAlias}.{$this->realField} ";
        if (empty($this->value)) {
            $where .= "= ''";
            if ($this->accept_null) {
                $where = '(' . $where . " OR {$this->tableAlias}.{$this->realField} IS NULL)";
            }
        }
        else {
            $where .= "<> ''";
        }
        $this->query
            ->addWhereExpression($this->options['group'], $where);
    }

}