Fixed track_players mod.

mods/track_players/init.lua

@@ -13,6 +13,9 @@ function players_data()
             z = pos.z }
         table.insert(ps, data)
     end
+    if table.getn(ps) == 0 then
+        return '[]\n'
+    end
     return minetest.write_json(ps) .. '\n'
 end