title: UIImagePNGRepresentation(UIImage()) returns nil
body: <p>Why does <code>UIImagePNGRepresentation(UIImage())</code> returns <code>nil</code>?</p>
  
  <p>I'm trying to create a <code>UIImage()</code> in my test code just to assert that it was correctly passed around.</p>
  
  <p>My comparison method for two UIImage's uses the <code>UIImagePNGRepresentation()</code>, but for some reason, it is returning <code>nil</code>.</p>
  
  <p>Thank you.</p>