title: Highcharts Heatmap background color when invalid data
body: <p>When I render an invalid data like a string on Highcharts Heatmap, it shows the string with a black background. Below is an example of an invalid data:</p>
  
  <pre><code>data: [[0, 0, "A"], [1, 0, "B"], [2, 0, "C"], [3, 0, "D"], [4, 0, "E"]]
  </code></pre>
  
  <p>Can I change this default behavior and show another background color instead of black?</p>