When a theme setting of type `upload` has a default upload, it should return the URL of the specified default upload until a custom upload is used for the setting. However, currently this isn't the case and we get null instead of the default upload URL.
The reason for this is because the `super` method of `#value` already returns the default upload URL (if there's one), so we can't pass that to `cdn_url` which expects an upload ID:
c961dcc757/lib/theme_settings_manager.rb (L212)
This commit fixes the bug by skipping the call to `cdn_url` when we fallback to the default upload for the setting value.
3f908c047d