def concat(str1: str, str2: str) -> str:
    """
    Returns the concatenation of 2 strings. A string
    with null value is considered as an empty string.
    """
    if str1 and str2:
        return "{} ~ {}".format(str1, str2)

    if not str1 and str2:
        return str2

    if str1 and not str2:
        return str1

    return ""