fix: Use correct post type when querying for ContentNodeSeo on revisions.

[idflood]

Implement fix for get_object_type() when object is a revision.

What

The function get_object_type returned null when using asPreview parameter.

Why

When using asPreview, the content type is revision and doesn't match the allowed types.

How

This pull request check if $this->data->post_type is a revision, and if this is the case it returns the parent post type.

Testing Instructions

You can try a query like this while being connected.

{
  contentNode(id: "2011", idType: DATABASE_ID, asPreview: true) {
    ... on Page {
      title

      seo {
          description
      }
    }
  }
}

The graphql error happen after adding asPreview parameter.

Checklist:

• My code is tested to the best of my abilities.
• My code follows the WordPress Coding Standards.
• My code has proper inline documentation.
• I have added unit tests to verify the code works as intended.
• The changes in this PR have been noted in CHANGELOG.md

[justlevine]

Thanks for this @idflood ! I've confirmed both the issue and this fix locally, and plan to add the missing unit test and get it merged over the weekend.

PS: this PR qualifies for my WPGraphQL Spring Cleaning campaign, where I'm donating dev hours to WPGraphQL projects for every issue/PR opened between now and April 31st. If you're interested, let me know via the link where you want the time donated to.