title: ng-repeat filter null value not displaying
body: <p>Why won't angular display values that are null when I apply:</p>
  
  <pre><code>ng-repeat="p in foo | filter: filter2"
  </code></pre>
  
  <p>where filter2 is </p>
  
  <pre><code> $scope.filter2 = function(p){
      if (p.state === null){
          return p.state;
      } else{
          return;
      }
  };
  </code></pre>
  
  <p>here's a plnkr with what I'm saying: <a href="http://plnkr.co/edit/cj3v1fuBu6sHVI7A4qlq?p=preview" rel="noreferrer">http://plnkr.co/edit/cj3v1fuBu6sHVI7A4qlq?p=preview</a></p>
  
  <p>The filter works when it's anything but a null but I'm trying to only the null ones to display. I could try changing all the null values to a default value string other than a null? Is there anywhere to get the filter to work with nulls?</p>