support an option to export empty png/jpg assets from transparent layer

Our customer manages an animation sequence on PSD files and each layer represents 1 frame of the animation.

The animation sequnce can have empty (transparent) layer because no image should be displayed for a few frames during animation.

In order to deliver the animation specification exported through Qt bridge to the development team, they hope image assets for the empty layer will also be extracted.

In developer's perspective, to make sequence animation by NumberAnimation or something, it is efficient that there are empty image assets are. 

We suggest adding an option in the form of a check box so that the image asset can be extracted even from the empty layer.