[MusicXML] import assumes beam=NONE if no beam information
Apparently the import process assumes no beam at all if there is no beam information in the score. It leads to some weird results out of the box. Like with this file from Notion 4 http://musescore.org/sites/musescore.org/files/test%20croches%20XML.xml
Maybe MuseScore could assume autobeam instead?