Fix tempo import from MIDI file

Fix http://musescore.org/en/node/37971 (again).
Previously MIDI import ignored empty tracks (tracks without notes), including tempo track that was incorrect.

See screenshots of the new import:
https://drive.google.com/file/d/0B5alKuFoSol2aFNGN1ZReDFNdVU/view?usp=sharing
https://drive.google.com/file/d/0B5alKuFoSol2dEVlbUNJMnlrMzA/view?usp=sharing