How to assign null for "object" type property in openapi v3 yaml file

Question

I am generating the code using openapi generator but in generated java files null is not assigned as default value. yaml file :

ClassA:
      type: object
      properties:
        classB:
          $ref: '#/components/schemas/ClassB'

generated java file :

ClassA.java
{
   private ClassB classB **= ;**
}

here it is not assigning null after =.