Don't set an empty track name in MIDI

At least old Solton/Ketron arrangers cannot deal with an explicit track title of "" in the control track and get mixed up enough that they refuse playing the whole MIDI file, making "" as the default track title a liability for users of those instruments.

Strictly speaking, it would be more correct to distinguish the cases of setting "" as track title explicitly (and pass it on) and not setting it at all (and omit a track name event) but it is not clear that it would be of actual value for anybody.