tileset.asset.fileName returns `null` in export_to_godot_tilemap.mjs

[calebtnorth]

I was having difficulty utilizing the relativePath property exporting a TileMap when, upon testing, realized it would properly set the ext_resource in the final .tscn if I specified the exact path to the tileset (e.g. /Assets/Maps/Town/town_map.tres).|

This did not happen with Tilesets, as the exported .tres properly linked to their source image without projectRoot or relativePath needing to be specified.

I dug around the source code only to find that tileset.asset.fileName in setTilesetsString() returned null when tileset.asset returned a proper object reference, resulting in relativePath only working if it included the tileset file name as well.