[MusicXML] Import can produce empty voice 1
MuseScore 1.1, current branch and trunk r4754
Finale produces MusicXML file with no voice 1 in some measures. When importing such a file, MuseScore ends up with no voice 1 in these measures. As MuseScore assume voice 1 is always present, it could lead to different problems.
See measure 7 to 9 in the attached file.