title: Expected null or template in return value from inclusion function when update to meteor 0.8
body: <p>I was following the instructions (<a href="https://github.com/meteor/meteor/wiki/Using-Blaze" rel="nofollow">https://github.com/meteor/meteor/wiki/Using-Blaze</a>) and trying to upgrade the project from meteor 0.7.2 to meteor0.8.1.3, but meet some errors and exceptions.</p>
  
  <p>Errors: </p>
  
  <pre><code>Uncaught Error: Expected null or template in return value from inclusion function, found: false localhost:3000/packages/spacebars.js?f49560fc90a8d71b7637d9596696078881b8c812:72
  </code></pre>
  
  <p>this errors comes from helper.js which has code as following:</p>
  
  <pre><code>UI.registerHelper('renderHeader', function () {
  
     if (settingsSubscription.ready()) {
         return Template.header;
    }else {
        return "";
    }
  });
  </code></pre>
  
  <p>and in index.html I use <em>{{renderHeader}}</em> to achive the template. I think the error means the function return a null template when settingSubscrition is not ready. So what template should I return when settingSubscription is not ready?</p>