Handle empty measures in a sane way

[florczakraf]

while we don't hang like SL parser, we produce something. I'm not really sure what's the best solution here, because there's also another related edge case -- when STEPS end with ,;

Example:

  6465 ,
  6466 0000
  6467 0000
  6468 0000
  6469 0000
  6470 ,
  6471 ,  // measure 16
  6472 0000
  6473 0000
  6474 0000
  6475 0000

should probably be treated as:

  6467 0000
  6468 0000
  6469 0000
  6470 ,
  6471 0000
  6472 ,  // measure 16
  6473 0000
  6474 0000
  6475 0000
  6476 0000
  6477 0000