fix sort for empty strings, fixes #167

oml/item/models.py

@@ -187,6 +187,8 @@ class Item(db.Model):
                             value = ox.sort_string(value).lower()
                 elif isinstance(value, list): #empty list
                     value = None
+                if not value and sort_type != 'boolean':
+                    value = None
                 if getattr(s, key['id']) != value:
                     setattr(s, key['id'], value)
                     update = True