title: Querying NOT NULL GraphQL with Prisma
body: <p>Schema:</p>
  
  <pre><code>type TrackUser {
    id: ID! @unique
    createdAt: DateTime!
    user: User #note there is no `!`
  }
  type User {
    id: ID! @unique
    name: String! @unique
  }
  </code></pre>
  
  <p>I want to get Alls <code>TrackUser</code> where <code>User</code> is not null. What would be the query?</p>