framework/behaviors/OptimisticLockBehavior.php
protected function getValue($event)
    {
        if ($this->value === null) {
            $request = Yii::$app->getRequest();
            $lock = $this->getLockAttribute();
            $formName = $this->owner->formName();
            $formValue = $formName ? ArrayHelper::getValue($request->getBodyParams(), $formName . '.' . $lock) : null;
            $input = $formValue ?: $request->getBodyParam($lock);
            $isValid = $input && (new NumberValidator())->validate($input);
            return $isValid ? $input : 0;
        }

        return parent::getValue($event);
    }
protected function getValue($event)
    {
        if ($this->value === null) {
            $request = Yii::$app->getRequest();
            $lock = $this->getLockAttribute();
            $formName = $this->owner->formName();
            $formValue = $formName ? ArrayHelper::getValue($request->getBodyParams(), $formName . '.' . $lock) : null;
            $input = $formValue ?: $request->getBodyParam($lock);
            $isValid = $input && (new NumberValidator())->validate($input);
            return $isValid ? $input : 0;
        }

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

In case of `null`, value will be parsed from [[\yii\web\Request::getBodyParam()|getBodyParam()]] or set to 0.