Null sprites should be displayed as white instead of transparent.

[Nixill]

When something is placed in the level but NecroEdit can't determine the sprite it should get, it displays absolutely nothing there instead, but it will still save that thing back into the level. This can mean that there's something there that shouldn't be and the person editing the level doesn't know about it. Therefore they should display as white squares (or red/yellow/green/blue depending on thing type - tile, enemy, pickup, trap)