Use "Unknown Artist" when tag is missing

This fixes the case where you switch from a file with an artist to one
with no artist, which would show the old artist.

The album field is already hidden, title shows the filename if missing,
and the track info screen is fully reset, so this covers everything
currently present.

lua/playing.lua

@@ -290,7 +290,7 @@ return screen:new {
         else
           album:add_flag(lvgl.FLAG.HIDDEN)
         end
-        artist:set { text = track.artist }
+        artist:set { text = track.artist or "Unknown Artist" }
       end),
       queue.position:bind(function(pos)
         if not pos then return end