Icon generates black/transparent when empty chunks are in view

If there are empty (completely air) chunks in view, when the game attempts to create an icon for the world, the icon will be black (transparent?).

Steps to reproduce:
1. Create a new world (easiest with Void Superflat)
2. Look at an area with missing or empty chunks
3. Press esc to trigger the icon generation
4. Save and quit, then look at the new icon generated.