How to give NULL instead of default_value in Symfony 4.3 entity (datetime field type)?

Question

symfony

MySQL/MariaDB base, datetime field with default value = "0000-00-00 00:00:00". According to the recommendations, NULL is not set, because with default_value, the speed of sampling from the database is faster.

//src/Entity/Customer.php

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="last_bill_date", type="datetime", nullable=false, options={"default"="0000-00-00 00:00:00"})
     */
    private $lastBillDate;

    /**
     * @return \DateTime
     */
    public function getLastBillDate(): \DateTime
    {
        return $this->lastBillDate;
    }

In the default_value case, it returns a DateTime('0000-00-00 00:00:00') object, given the time zone.
Further, problems naturally arise, for example, in Twig a date like -0001-11-30 00:00:00 with type="datetime" in getLastBillDate(), a ready-made DateTime comes:

//src/Entity/Customer.php

    /**
     * @ORM\Column(name="last_bill_date", type="string")
     */
    private $lastBillDate;

    /**
     * @return null|\DateTime
     */
    public function getLastBillDate(): ?\DateTime
    {
        $result = $this->lastBillDate !== '0000-00-00 00:00:00' ? new \DateTime($this->lastBillDate) : NULL;

        return $result;
    }

Tell me how to do it right