When metadata is absent, the %S template variable does not have a default value set in the dictionary, which can cause a "KeyError: 'S'" error. While %A defaults to 'Unknown Artist' and %S might conventionally default to 'Artist, Unknown', defaulting %S to that is unnecessary, because manual tagging typically follows to assign the correct name. Therefore, I default %S to 'Unknown Artist', just like %A. Signed-off-by: sinofp <sinofp@tuta.io>