Account for tilesets that do not give a tileoffset.

My test map didn't have any tile offsets, and thus was crashing:

    Error: sti/map.lua:87: attempt to index field 'tileoffset' (a nil value)

This is my proposed fix. In case you want to se the relevant .lua section of my map:

```lua
  tilesets = {
    {
      name = "isometric_grass_and_water",
      firstgid = 1,
      tilewidth = 64,
      tileheight = 64,
      spacing = 0,
      margin = 0,
      image = "isometric_grass_and_water.png",
      imagewidth = 256,
      imageheight = 384,
      properties = {},
      tiles = {}
    }
  },
```