title: How can I declare a PropType corresponding to a nullable number?
body: <p>I'm looking for a <code>PropType</code> that means </p>
  
  <blockquote>
    <p>"this is required, and it will either be a number or be null"</p>
  </blockquote>
  
  <p>In other words, what I have now is </p>
  
  <pre><code>PropTypes.number.isRequired
  </code></pre>
  
  <p>but that throws a warning if a <code>null</code> value gets passed in, yet I want <code>null</code> to be an acceptable value.</p>