fix 750: No warning for non-found voice in lyrics combine when lyrics are empty

fix 750: No warning for non-found voice in lyrics combine when lyrics are empty

Use a bool flag to store whether the Lyrics context was ever really created
(i.e. one moment was processed). If that's not the case, then the lyrics
are empty, and we shouldn't print any warning about non-existing voices
(since we never even tried to find an appropriate voice; and we also don't
need one anyway).

Fixes isse 750 (http://code.google.com/p/lilypond/issues/detail?id=750)