When creating a Photoshop file with only group layers we cannot open the file

[EmilDohne]

This appears to be due to the way we calculate the amount of channels in a file when going from LayeredFile<T> -> PhotoshopFile. It doesnt detect any channels and therefore writes out 0 which isnt valid for Photoshop. What it appears to actually do is just write out 4 channels in RGB mode and write that into the merged image data section

[EmilDohne]

In addition we default initialize the LayeredFile document with a width and height of 0 which isnt valid. This should be set to 1 by default