在字段时间mysql中保存空数据

I need to insert null data if so come. I'm inserting data from EXCEL to my DB. The problem is that I made the table structure and field to allow nulls and also as a default to NULL.

The value is empty and pass all my logical validations but when inserted, left in 21:00:00

PHP :

if(!empty($r['tiempo_chip'])){
    $resultados[$i]['tiempo_chip'] = $this->ExcelReader->timeFormat($r['tiempo_chip']);
}else{
    $resultados[$i]['tiempo_chip'] = NULL;
}

Data :

[0] => Array
    (
        [numero] => 88
        [tiempo_oficial] => 01:10:30
        [tiempo_chip] => 
    )

This is stored in the DB 21:00:00

CREATE TABLE IF NOT EXISTS `resultados` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`numero` int(11) NOT NULL,
`tiempo_oficial` time NOT NULL,
`tiempo_chip` time DEFAULT NULL,
`created` datetime NOT NULL,
`modified` datetime NOT NULL,
)  ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=133 ;