framework/behaviors/BlameableBehavior.php
protected function getValue($event)
    {
        if ($this->value === null && Yii::$app->has('user')) {
            $userId = Yii::$app->get('user')->id;
            if ($userId === null) {
                return $this->getDefaultValue($event);
            }

            return $userId;
        } elseif ($this->value === null) {
            return $this->getDefaultValue($event);
        }

        return parent::getValue($event);
    }
protected function getValue($event)
    {
        if ($this->value === null && Yii::$app->has('user')) {
            $userId = Yii::$app->get('user')->id;
            if ($userId === null) {
                return $this->getDefaultValue($event);
            }

            return $userId;
        } elseif ($this->value === null) {
            return $this->getDefaultValue($event);
        }

        return parent::getValue($event);
    }
{@inheritdoc}

In case, when the [[value]] property is `null`, the value of [[defaultValue]] will be used as the value.