title: Gatsby with wordpress does not find an ACF flexible content field if it is unused
body: <p>I have a flexible content field type in Wordpress (with ACF) and I am getting errors when trying to build gatsby.</p>
  
  <p>I use the following plugins:</p>
  
  <ul>
  <li>ACF to REST api</li>
  <li>ACF to REST api recursive</li>
  <li>Advanced Custom Fields PRO</li>
  </ul>
  
  <p>For gatsby I use gatsby-source-wordpress.</p>
  
  <pre><code>{
    allWordpressPage {
      edges {
        node {
          title
          acf {
            page_builder_page {
              ... on WordPressAcf_hero {
                title
                subtitle
              }
              ... on WordpressAcf_text {
                text
              }
            }
          }
        }
      }
    }
  }
  </code></pre>
  
  <p>A query like the one above does work, only if the block <em>page_builder</em> on the <em>page</em> type for some page is using the <em>hero</em> and the <em>text</em> block. If I set up this page for the first time, or create a new custom post type with the same page_builder field, I need to fill at least one field for every flexible content block type before the graphql-query will work.</p>
  
  <p>Otherwise I get errors similar to this for each unused block (if hero-type has content but not text-type for instance):</p>
  
  <pre><code>GraphQL request: Fragment "TextBlockFragment" cannot be spread here as objects of
  type "WordPressAcf_hero" can never be of type "WordPressAcf_text".
  </code></pre>
  
  <p>Is there a solution to this? I guess it is not supposed to be this way. As it is now, I need to fill the whole page with dummy content the first time it is set up before I can actually build it with Gatsby.</p>