title: Loopback4 and MongoDB query documents with null or undefined field
body: <p>How do I query for document with fields that are null or undefined?</p>
  
  <p>e.g. </p>
  
  <pre><code>{id:1, updatedAt: null}
  {id:2}
  </code></pre>
  
  <p>I already tried this code to no avail. Returns 0 matches.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>const whereBuilder = new WhereBuilder();
  const where: Where = whereBuilder
    .eq('updatedAt', null)
  //.eq('updatedAt', false)
    .build();
  
  myRepository.findOne({
    where: where,
  });</code></pre>
  </div>
  </div>
  </p>
  
  <p>Thanks for the Help!</p>
  
  <p>Edit 1: Field declaration on model.</p>
  
  <pre><code>@property({
    type: 'date',
  })
  updatedAt?: string;
  </code></pre>