title: GraphQL "Cannot return null for non-nullable"
body: <p>Trying to make my first graphQL server, here's what I have written so far.</p>
  
  <p><a href="https://gist.github.com/tharakabimal/7f2947e805e69f67af2b633268db0406" rel="noreferrer">https://gist.github.com/tharakabimal/7f2947e805e69f67af2b633268db0406</a></p>
  
  <p>Following error pops up on GraphQL when I try to filter the users by username.</p>
  
  <p><a href="https://i.stack.imgur.com/HQSqz.png" rel="noreferrer">Error on GraphQL</a></p>
  
  <p>The error occurs in the users field in UserQueriesQL.js. </p>
  
  <p>Is there anything wrong the way I pass arguments on the resolve functions?</p>
  
  <pre><code>user: {
  type: UserType,
  args: { 
    username: {
      name: 'username',
      type: new GraphQLNonNull(GraphQLString)
    }
  },
  resolve: function(parentValue, args) {
    return User.find( args ).exec();
  }
  </code></pre>