def listToStr(in_list):
    """Return string with comma separation from list
    
    Parameters
    ----------
    in_list : list                          
      List to merge and comma delineate
    
    Returns
    -------
    str or None
      Comma delineated string or None if input is invalid
    """
    if len(in_list) > 1:
        return ', '.join(in_list)
    elif len(in_list) == 1:
        return in_list[0]
    else:
        return None