[BAC-640] Move $ref in allOf for Entities when nullable

[taleh007]

it will help to generate proper schema

Current json schema cannot be used to generate nullable reference in sdk:

class HistoryEntry < APIEntity
  attribute :answer, :entity, class_name: 'Ask::Messages::Answer'
  attribute :answers, :array, of: 'Ask::Messages::Answer'

  validates :answer, presence: { allow_nil: true }
  validates :answers, presence: { allow_nil: true }
end

swagger (3.0.1)=>

HistoryEntry:
    type: object
    required:
    - answer
    - answers
    properties:
    answer:
        "$ref": "#/components/schemas/Ask.Messages.Answer"
        nullable: true
    answers:
        items:
        - "$ref": "#/components/schemas/Ask.Messages.Answer"
        type: array
        nullable: true

openapi-ts =>

export type HistoryEntry = {
    answer: AskMessagesAnswer;
    answers: Array<AskMessagesAnswer> | null;
};

it should be:

export type HistoryEntry = {
    answer: AskMessagesAnswer | null;
    answers: Array<AskMessagesAnswer> | null;
};