message: GeoJsonTiler fix for empty tiles
  `geojsonvt` returns `null` for tiles with no data, so ensure we
  return empty object in this case.
diff: @@ -73,6 +73,6 @@ export class GeoJsonTiler implements ITiler {
  if (index === undefined) {
  throw new Error("Tile not found");
  }
  - return index.getTile(tileKey.level, tileKey.column, tileKey.row);
  + return index.getTile(tileKey.level, tileKey.column, tileKey.row) || {};
  }
  }
  