Nak.  Needs to be:

  if (tag.getTagger() != null) {
    w.write("tagger ");
    w.write(tag.getTagger().toExternalString());
    w.write('\n');
  }

That is we only produce the "tagger" header line if there is an identity that will appear on it.  We never should create an empty header line.

I'll fix it during cherry-pick.