avoid passing nil to string.sub

mpris-widget/init.lua

@@ -50,10 +50,13 @@ function playerctl:watch(timeout, callback, widget)
             position = position,
             length = length,
             album = words[7],
-            year = string.sub(words[8], 0, 4),
             progress = progress,
         }
 
+        if words[8] ~= nil then
+            metadata.year = string.sub(words[8], 0, 4)
+        end
+
         callback(widget, metadata)
     end