UIImagePNGRepresentation(UIImage()) returns nil

UIImagePNGRepresentation() will return nil if the UIImage provided does not contain any data. From the UIKit Documentation:

Return Value

A data object containing the PNG data, or nil if there was a problem generating the data. This function may return nil if the image has no data or if the underlying CGImageRef contains data in an unsupported bitmap format.

When you initialize a UIImage by simply using UIImage(), it creates a UIImage with no data. Although the image isn't nil, it still has no data. And, because the image has no data, UIImagePNGRepresentation() just returns nil.

To fix this, you would have to use UIImage with data. For example:

var imageName: String = "MyImageName.png"
var image = UIImage(named: imageName)
var rep = UIImagePNGRepresentation(image)

Where imageName is the name of your image, included in your application.

In order to use UIImagePNGRepresentation(image), image must not be nil, and it must also have data.

If you want to check if they have any data, you could use:

if(image == nil || image == UIImage()){
  //image is nil, or has no data
}
else{
  //image has data
}

The UIImage documentation says

Image objects are immutable, so you cannot change their properties after creation. This means that you generally specify an image’s properties at initialization time or rely on the image’s metadata to provide the property value.

Since you've created the UIImage without providing any image data, the object you've created has no meaning as an image. UIKit and Core Graphics don't appear to allow 0x0 images.

The simplest fix is to create a 1x1 image instead:

UIGraphicsBeginImageContext(CGSizeMake(1, 1))
let image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()