GraphQL error when returning array fields in cloud function

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

Returning Parse Objects with array fields from a cloud function using GraphQL results in a runtime error.

In our example (simplified for clarity) we have added an array field roles to our Parse.User class. This has never been an issue with regular Parse queries or with autogenerated GraphQL queries.

Now we have been trying to add a cloud function search that returns an array of Parse.User, but we get following error when calling it with GraphQL:

"Abstract type "ArrayResult" must resolve to an Object type at runtime for field "User.roles". Either the "ArrayResult" type should provide a "resolveType" function or each possible type should provide an "isTypeOf" function."

Cloud function

async function search(request) {
  return new Parse.Query(Parse.User)
    .exists('roles')
    .find({ useMasterKey: true });
}

GraphQL schema

extend type Query {
  search(term: String!): [User] @resolve
}

GraphQL query

query userSearch {
  search(term: "test") {
    objectId
    username
    roles {
      ... on Element {
        value
      }
    }
  }
}

Example User object

{
  objectId: 'IeZkxfGtky'
  username: 'sana',
  roles: [ 'verified', 'moderator' ],
  // snip
}

Server

• Parse Server version: 4.5.0

Start by reproducing the error with the shown search cloud function, schema, query, and Parse.User roles array on Parse Server 4.5.0. Trace how GraphQL resolves the User.roles ArrayResult field for objects returned by a cloud function. Done means the query returns array fields without the abstract-type runtime error.