Custom relation entity missed translation feature on V2.1

[tianyingchun]

Describe the bug

Please consider below sense

1. we have custom fields on Collection entity named campaign with type: relation, eager:true

/**
 * `Collection` basic custom fields for `campaign`
 */
export const collectionCustomFields: CustomFieldConfig[] = [
  {
    type: 'relation',
    name: 'campaign',
    nullable: true,
    entity: Campaign,
    eager: true,
    public: true,
    label: [
      {
        languageCode: LanguageCode.en,
        value: 'Campaign',
      },
    ],
    description: [
      {
        languageCode: LanguageCode.en,
        value: 'Campaign of this collection page',
      },
    ],
  },
];

2. for campaign.entity we have translation feature defined campaign.translaton.entity
3. we defined a gql schema for compaign as below

  type Campaign implements Node {
    id: ID!
    createdAt: DateTime!
    updatedAt: DateTime!
    name: String
    code: String!
   "this field will be include on `CampaignTranslation`"
    languageCode: LanguageCode!
    translations: [CampaignTranslation!]!
  }

4. cause of languageCode only defined on CampaignTranslation.entity
5. go to admin-ui, nagivate to catalog/collections, click to children catalog item

query GetCollectionList($options: CollectionListOptions)

it will throw error:

"message": "Cannot return null for non-nullable field Campaign.languageCode."
becuase

...
 customFields {
        collectionType
        isFeatured
        invisible
        metaTitle
        metaDescription
        collectionKeys
        campaign {
           id 
          // this field need to be `translation`
          languageCode
          ...
       } 
}

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Environment (please complete the following information):

• @vendure/core version: 2.1
• Nodejs version: 18
• Database (mysql/postgres etc):mysql

Additional context
Add any other context about the problem here.

[tianyingchun]

the mini repo is :https://github.com/tianyingchun/vendure-issue.git

1. checkout branch vendure_issure_2453
2. yarn install
3. goto plugin-issue
4. npm run build
5. goto dev-server
6. yarn serve
7. http://localhost:3001/admin/catalog/collections
   it will throw error.