'Set None for nullable fields that has unknown value.
In case model attribute is not present in database, value stored under
``attrname\'\' field will be unknown. In such cases if the field
``attrname\'\' is a nullable Field return None'

def obj_load_attr(self, attrname):

    try:
        is_attr_nullable = self.fields[attrname].nullable
    except KeyError:
        return super(NeutronDbObject, self).obj_load_attr(attrname)
    if is_attr_nullable:
        self[attrname] = None
