import re


def clean_string(target_str):
    """ Remove ISO control characters and trim input string. Returns None if cleaned string is empty.

    Args:
        target_str (st): string to be cleaned.

    Returns:
        str: cleaned input string.
    """
    if target_str is None:
        return None
    else:
        string_clean = re.sub(r'[\x00-\x1F]+', '', target_str).strip()
        if string_clean == '':
            return None
        else:
            return string_clean