to_python returns list instead of None

multiselectfield/db/fields.py

@@ -107,6 +107,7 @@ class MultiSelectField(models.CharField):
     def to_python(self, value):
         if value:
             return value if isinstance(value, list) else value.split(',')
+        return []
 
     def contribute_to_class(self, cls, name):
         super(MultiSelectField, self).contribute_to_class(cls, name)