ratings and scores use 0 for default value

[Caleb Cushing]

https://bugs.kde.org/show_bug.cgi?id=172112

unrated and unscored should be null values in the database, not having
them as null creates a problem when ever you want to set up a playlist
that includes greater than certain ratings or scores. and unrated
track should not be considered less than a rated track.

sql wise this is as simple as making sure the default value when
creating tracks is null.
-- 
Caleb Cushing