def format_sex_string(sex_str):
    """
    Converts 'M', 'F', or else to 'male', 'female', or empty string (e.g. '').

    Args:
        sex_str (str): String consisting of 'M', 'F', '', or None.

    Returns:
        str: 'M', 'F', or ''
    """
    if sex_str == "M":
        sex = "male"
    elif sex_str == "F":
        sex = "female"
    else:
        sex = ""
    return sex