message: Fix FilterContext to return None if it fails to find value
diff: @@ -56,7 +56,10 @@ impl<'a> FilterContext<'a> {
  if &target.table == alias || table == name {
  get_value()
  } else {
  - self.next.unwrap().get_value(target)
  + match self.next {
  + Some(c) => c.get_value(target),
  + None => None,
  + }
  }
  }
  }
  