[6.x] Add missing nullable type declaration for the model timestamp fields

[antonkomarev]

This PR replaces #34785 targeted to 8.x because @GrahamCampbell said to target this changes to 6.x

Eloquent Model constants CREATED_AT & UPDATED_AT are nullable. There are checks for null values, but null is not documented in PHPDoc.

public function updateTimestamps()
{
    $time = $this->freshTimestamp();

    $updatedAtColumn = $this->getUpdatedAtColumn();

    if (! is_null($updatedAtColumn) && ! $this->isDirty($updatedAtColumn)) {
        $this->setUpdatedAt($time);
    }

    $createdAtColumn = $this->getCreatedAtColumn();

    if (! $this->exists && ! is_null($createdAtColumn) && ! $this->isDirty($createdAtColumn)) {
        $this->setCreatedAt($time);
    }
}