What is equavalent to allowEmpty=>true in Yii 2.0

Yii 2.0 General Discussions
1

In Yii1.1 I was using the following validation rule for optional fields.

"length","allowEmpty"=>true

Can anybody tell me the equivalent code for Yii 2 ????

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Found the solution:

[[‘attribute’],‘default’,‘value’=>’’],

2

‘skipOnEmpty’=>true

SkipOnEmpty Docs