title: Null Values in Zones in Highcharts
body: <p>I have graphs that have zones. Currently if there is a null value, that null value is displayed in the tooltip with the zone color of the point before it. I want the null value to always be the color black.</p>
  
  <p>For example, 
  <a href="https://i.stack.imgur.com/F9bOs.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/F9bOs.png" alt="Zones"></a></p>
  
  <p>The tooltip in graph 1 has all the correct information... but "Strength: null" has a green dot. I want to make this dot black because it is a null value.</p>
  
  <p>Is it possible to make this happen? Thanks so much.</p>
  
  <p><a href="https://codepen.io/austeng/pen/gOppRWY" rel="nofollow noreferrer">https://codepen.io/austeng/pen/gOppRWY</a></p>
  
  <pre><code>zones: [
    {
      value: 0.3,
      color: "green"
    },
    {
      value: 0.5,
      color: "black",
    },
    {
      color: "red"
    }
  ]
  </code></pre>