我也遇到同样的问题，处理方法如下：

namespace app\models;

use Yii;
use yii\db\ActiveRecord;

class Page extends ActiveRecord
{	

    public function rules()
    {
        return [
            [['title', 'img_url'], 'required'],
            [['img_url'], 'file', 'skipOnEmpty' => true, 'extensions' => 'png, jpg'], //这里是关键 skipOnEmpty 设为true
        ];
    }


    public function upload()
    {
        if ($this->validate()) {
        	$file_path = 'uploads/' . time() . $this->img_url->extension;
        	$this->img_url->saveAs($file_path);
            return $file_path;
        } else {
            return false;
        }
    }
}

action里写法

    public function actionIndex()
    {
        $model = new Page();
        if (Yii::$app->request->isPost && $model->load(Yii::$app->request->post())) {
            $model->img_url = UploadedFile::getInstance($model, 'img_url');
            $file_path      = $model->upload();
            if ($file_path !== false) {
                $model->img_url  = $file_path;
                $model->add_date = time();
                // 文件上传成功
                if ($model->save()) {
                    $this->redirect(array('step/show', 'id' => $model->id));
                }
            }
        }
        return $this->render('index', [
            'model' => $model,
        ]);
    }