convert empty voxels into solid voxels

there should be no way to encode air voxels in voxelbuilder creations, air is just the lack of data, not the presence of a 0. this fixes a bug with green voxels, which come decode as value 0 for some reason, causing this:

![screen shot 2013-08-14 at 2 55 03 pm](https://f.cloud.github.com/assets/39759/965512/3f1fcf2a-052c-11e3-8307-f41893786bd7.png)

after this fix:

![screen shot 2013-08-14 at 2 54 37 pm](https://f.cloud.github.com/assets/39759/965515/42ffef1c-052c-11e3-9a77-fc62dbe19203.png)