PlayerThread: Only drop 0 length packets without tags

Fixes a regression from 752dfb3d95 which
caused the current chunk to be flushed as soon as new replaygain
information was found. If this occurs on a tag chunk, it has no data
(length 0) and is then skipped before pushing it to all of the outputs.

This change allows 0-length chunks through if they contain a tag and
they are now appearing in mplayer and mpv properly.

src/PlayerThread.cxx

@@ -724,7 +724,7 @@ play_chunk(player_control &pc,
 	if (chunk->tag != nullptr)
 		update_song_tag(song, *chunk->tag);
 
-	if (chunk->length == 0) {
+	if (chunk->IsEmpty()) {
 		buffer.Return(chunk);
 		return true;
 	}