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>
cac1fe5ed4