makeOptionalNode() {
    if (this.chainContext) {
      this.chainContext.countChoiceContexts += 1;
      this.pushChoiceContext('??', false);
    }
  }
Create a choice context for optional access.
This method is called on entering to each `(Call|Member)Expression[optional=true]` node.
This creates a choice context as similar to `LogicalExpression[operator="??"]` node.
@returns {void}