World Icon (sometimes) isn't created for nearly empty worlds

Affects:

• Superflat Void Preset

In order to create the icon there should be 11 close cubic chunks with blocks on the screen (Found via experiments). So, if there are less nonempty chunks in the world (e.g. Void preset has only 9 chunks by default), it will never get the icon automatically. Even if there are 11 and more those chunks there, the icon won't be created unless you look at least at 11 of them at the same time, which is sometimes impossible (e.g. they are too far from each other) or rarely achieved (e.g. you need to fly higher and then look down)