options.lua: avoid unnecessary on_update calls

The script was set up to only call on_update when the changelist
was non-empty. However, since the size operator does not operate
on dicts, it always returned 0 (which is truthy), thus on_update
would always be called when the script-opts property changed.

player/lua/options.lua

@@ -152,7 +152,7 @@ local function read_options(options, identifier, on_update)
                 end
             end
             last_opts = new_opts
-            if #changelist then
+            if next(changelist) ~= nil then
                 on_update(changelist)
             end
         end)