[@urql/exchange-graphcache] return null if a field is a union

[lavolpecheprogramma]

Describe the bug

Hi, i have a problem with graphcache when the requested content contains a union field. i created a reproduction link, and here you can find the graphql schema

the query is

query ExampleQuery($slug: String) {
  GetUnionTest(slug: $slug) {
    id
    title
    unionField {
      ... on ReferenceModel {
        __typename
        id
        title
      }
      ... on AccessibleImage {
        __typename
        id
        alt
        image {
          metaData
          originalName
        }
      }
    }
  }
}

i published 2 contents queriable by slug: 'content-with-component' and 'content-with-reference' The network response is ok with the right content, but urql returns null.

Thanks in advance

Reproduction

https://stackblitz.com/edit/github-54uamg

Urql version

"@urql/exchange-graphcache": "^7.0.0", "@urql/vue": "^1.2.1",

Validations

• [X] I can confirm that this is a bug report, and not a feature request, RFC, question, or discussion, for which GitHub Discussions should be used
• [X] Read the docs.
• [X] Follow our Code of Conduct

[lavolpecheprogramma]

amazing work! thank you very much! @JoviDeCroock