def add_default_tuint(table_hdu):
    """
    Add a default empty string tunit if not already defined

    Parameters
    ----------
    table_hdu : `astropy.io.fits.BinTableHDU`
        Binary table to add tunit to

    Returns
    -------
    `astropy.io.fits.BinTableHDU`
        Table with added tunits kewords
    """
    for col in table_hdu.columns:
        if not col.unit:
            col.unit = ' '
    return table_hdu