def hex_to_binary_str(hex_str: str):
    """Convert hex string to binary string

    @param hex_str: the hex string to convert
    @return: the binary string or None if the input is not a hex string
    """
    try:
        return bin(int(hex_str, 16))[2:].zfill(len(hex_str) * 4)
    except ValueError:
        pass