title: Graphql, return result only if record not null
body: <p>Totally new in Gatsby (my app is build in here). Also new in Graphql but I go this project from a colleague and now I need to find a way to sort it out.</p>
  <p>I have a query in graphql that looks as follow:</p>
  <pre><code>query MediasQuery {
      allStrapiMedias(sort: { fields: [date], order: DESC }) {
        edges {
          node {
            id
            name
            description
            date(formatString: &quot;Do MMMM, YYYY&quot;)
            link
            poster {
              childImageSharp {
                fluid {
                  src
                }
              }
            }
            file {
              url
            }
          }
        }
      }
    }
  </code></pre>
  <p>The issue I am facing is that the <code>file</code> record can exist or not, to be more accurate, if <code>link</code> record is empty, then I should find a <code>file</code> (this is how it's been build in Strapi).</p>
  <p>If I have at least one record with <code>file</code> then the query run normally, but as soon as I deleted this record from database all the app crash.</p>
  <p>How can I make that query to only return <code>file</code> if <code>url</code> is empty or null?</p>