title: hide the NA values when using display_numbers function in pheatmap
body: <p>I am plotting heatmap by <code>pheatmap</code> package in r.</p>
  
  <p>I applied the <code>display_numbers</code> function to display the values in a matrix into the heatmap, and I got:</p>
  
  <p><a href="https://i.stack.imgur.com/1I3BY.png" rel="nofollow noreferrer">heatmap</a></p>
  
  <p>I got so many NA in my matrix and I would like to hide them in the heatmap, how can I do that?</p>