def METHOD_NAME(out_format):
    """
    Helper function to get the output extension for the given output format.
    Parameters
    ----------
    out_format : str
        Output format name.
    Returns
    -------
    str : file extension without dot or empty if not supported
    """
    if (out_format is None) or (out_format is True):
        logger.warning('Dump to XLSX format by default')
        return 'xlsx'
    if out_format in output_formats:
        return output_formats[out_format][0]
    else:
        logger.error(f"Dump format <{out_format}> not supported.")
        return ''