Angular If Statement

I am new to Angular and I would like to know how to do something similar to an if statement.

I am using ionic to build my web app and have an xml feed coming in with a thumbnail which is shown like this "{{post.thumbnail}}"

Here is what I tried to do to show an alternative image if the image is empty.

<div ng-hide="{{post.thumbnail}} == null">
  <img src="{{post.thumbnail}}" width="100" height="100" alt=""/>
</div>
<div ng-hide="{{post.thumbnail}} != null">
  <img src="img/dining.jpg" width="100" height="100" alt=""/>
</div>