def stringify_col(df, col_name):
    """
    Take a dataframe and string-i-fy a column of values.
    Turn nan/None into "" and all other values into strings.

    Parameters
    ----------
    df : dataframe
    col_name : string
    """
    df = df.copy()
    df[col_name] = df[col_name].fillna("")
    df[col_name] = df[col_name].astype(str)
    return df