title: Angular: Reinclude null values when filter parameter is empty
body: <p>I have a pretty simple <code>textbox</code> filtering an <code>ng-repeat</code> on some unordered <code>li</code>s. When I add a value to the <code>textbox</code> the items with the <code>null</code> values are removed and do not return even when the <code>textbox</code> is cleared. I have an idea of <em>why</em> this is happening (the search object now has an empty property which doesn't match the <code>nulls</code>), but I cannot figure out how to solve the problem. I've tried to <code>pop()</code> the property off of the search object with no luck.</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;div ng-controller="ListCtrl"&gt;
      &lt;input type="text" ng-model="search.age" placeholder="Age"&gt;&lt;/input&gt;
      &lt;ul&gt;
          &lt;li ng-repeat="item in items | filter:search"&gt;
              {{item.name}} - {{item.age}}
          &lt;/li&gt;
      &lt;/ul&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>JS:</p>
  
  <pre><code>function ListCtrl($scope) {
    $scope.items = [
      {'name':'Carl', 'age':69},
      {'name':'Neil', 'age':54},
      {'name':'Richard'},
      {'name':'Chris', 'age':58}
    ];
  }
  </code></pre>
  
  <p>Please checkout the <a href="http://jsfiddle.net/jesseflorig/9Tpcw/" rel="nofollow noreferrer">JSfiddle</a> to better illustrate the issue.</p>