is there a way to define a filter for those songs that have either a high rating (>50) or have no rating set? so I could rate those songs low (<50) that I don't want to hear.


You can do a filter with:
Any of:
  rating set to default
  rating > 50%

You can also get this filter by typing "rating= OR rating>50" into the SimpleSearch widget (I'll document the syntax one day).
By the way there is a totally undocumented virtual field "ratingnumber" that always returns a number, using the default rating if the rating is not set.