OSBA EXT output without MD

The OSBA EXT output generates ISM metadata files though no metadata are actually transmitted, so only default values are written to the external file.

I suggest to follow the OMASA logic for which the EXT output is supported in the DISC mode only. See the logic in doSanityChecks_IVAS():

    if ( st_ivas->ivas_format == MASA_ISM_FORMAT )
    {
        if ( st_ivas->ism_mode != ISM_MASA_MODE_DISC && output_config == AUDIO_CONFIG_EXTERNAL )
        {
            return IVAS_ERROR( IVAS_ERR_INVALID_OUTPUT_FORMAT, "Incorrect output configuration specified for combined MASA and ISM format" );
        }
    }