title: ng-repeat filter for where an attribute is undefined
body: <p>I was hoping this would be really simple in angular. </p>
  
  <p>I have an list with translators and other users</p>
  
  <pre><code>[{user: 'a',languages:['french','english','spanish']},
  {user: 'b'}]
  </code></pre>
  
  <p>I then have an ng-repeat where I only want to display the users who have languages</p>
  
  <pre><code>ng-repeat="translator in users | filter: {languages}"
  </code></pre>
  
  <p>but unfortunately, I can't seem to figure out how to filter by the existence of an attribute. 
  I've tried all manner of </p>
  
  <pre><code>{languages:'!undefined'}
  {languages.length &gt; 0}
  {languages.length:'!0'}
  </code></pre>
  
  <p>I don't want to have to write an external filter for this as I'm sure it's possible in angular, I'm just not able to get the lingo right. </p>