'Return True if the value of field is null in this object.
\'null\' means it is unknown or evaluates to False. So a title of
_(\'Unknown\') is null or a language of \'und\' is null.
Be careful with numeric fields since this will return True for zero as
well as None.
Also returns True if the field does not exist.'

def is_null(self, field):

    try:
        null_val = NULL_VALUES.get(field, None)
        val = getattr(self, field, None)
        return ((not val) or (val == null_val))
    except:
        return True
