Fix PHP notices on orders data store

[Aljullu]

Fixes #1862.

Fixes PHP notices appearing in some queries to /reports/orders.

Notices

PHP Warning:  Division by zero in /wc-admin/includes/data-stores/class-wc-admin-reports-orders-data-store.php on line 180
PHP Notice:  Undefined property: stdClass::$pages in /wc-admin/includes/api/class-wc-admin-rest-reports-orders-controller.php on line 80
PHP Notice:  Undefined property: stdClass::$page_no in /wc-admin/includes/api/class-wc-admin-rest-reports-orders-controller.php on line 82
PHP Notice:  Undefined property: stdClass::$pages in /wc-admin/includes/api/class-wc-admin-rest-reports-orders-controller.php on line 83

Detailed test instructions:

• Set define( 'WP_DEBUG_LOG', true ); in your wp-config.php file, so error logs are stored in wp-content/debug.log (it might depend on your WP installation, though, for me they were stored in /var/log/php-fpm/www-error.log).
• Go to WooCommerce > Dashboard.
• Verify the notices described above do not appear anymore in the error log.

[ryelle]

Errors are gone, thanks! 👍