Xform index_val so that it can be stored/queried.

def map_index_val(index_val):
        """Xform index_val so that it can be stored/queried."""
        if index_val is None:
            return DynamoMappings.NONE_VAL

        index_val = str(index_val)
        if not index_val:
            return DynamoMappings.EMPTY_STR_VAL

        return index_val