title: ko.utils.arrayFirst always returns null when not handling else block with non-empty string
body: <p>This works correctly:</p>
  
  <pre><code>  self.getById = function(id) {
          return ko.utils.arrayFirst(self.PostArray(), function(item) {
              if (item.postId === id) {
                  return item;
              }
              else {
                  return 'not found';
              }
          });
      };
  
      console.log(self.PostArray().length);
      console.log(self.getById(170));
  </code></pre>
  
  <p>But if I put <code>return ''</code> or <code>return null</code> in else block I always get null, why is that?</p>