Relationship field query errors out when Post that it is set to is deleted

[dChiamp]

Related to issues #85, #100.

Say you have a post relationship field called "Editor Credit" in a Post custom field group ("Work Meta").
If a selected post within that relationship field is subsequently deleted, the query will return a 500 error.
This is difficult to debug because the relationship field that held the deleted post appears empty, but the error persists.
I would instead expect the system to return null (which it does when logged in) and give no errors.

Example query:

query PAGES($uri: String!) {
  nodeByUri(uri: $uri) {
    ... on Page {
      title
      id
      uri
      workMeta {
        editorCredit {
          ... on Page {
            title
            id
          }
        }
      }
    }
  }
}

Results in 500 "Internal server error"

Tested on:

• WordPress 5.8.1
• Advanced Custom Fields 5.10.2
• WP GraphQL 1.6.7
• WPGraphQL for ACF 0.5.2