Fix null bounding box being returned

Its usage is not always checked for null, so it is safer to return a
full block.

src/main/java/dan200/computercraft/shared/common/BlockGeneric.java

@@ -204,7 +204,7 @@ public final AxisAlignedBB getBoundingBox( IBlockState state, IBlockAccess world
             TileGeneric generic = (TileGeneric)tile;
             return generic.getBounds();
         }
-        return NULL_AABB;
+        return FULL_BLOCK_AABB;
     }
 
     @Override
@@ -237,7 +237,7 @@ public final AxisAlignedBB getCollisionBoundingBox( IBlockState state, World wor
                 return aabb;
             }
         }
-        return NULL_AABB;
+        return FULL_BLOCK_AABB;
     }
 
     @Override