How to use the node-opcua-secure-channel.SecurityPolicy.Invalid function in node-opcua-secure-channel

* or signing. If this SecurityPolicy is omitted then the Client uses the SecurityPolicy in the
     * EndpointDescription. If the matching SecurityPolicy is set to None then no encryption or signature
     * is required.
     *
     */
    const userTokenPolicy = findUserTokenPolicy(endpoint_desc, UserIdentityTokenType.USERNAME);

    // istanbul ignore next
    if (!userTokenPolicy) {
        throw new Error("Cannot find USERNAME user token policy in end point description");
    }

    let securityPolicy = fromURI(userTokenPolicy.securityPolicyUri);

    // if the security policy is not specified we use the session security policy
    if (securityPolicy === SecurityPolicy.Invalid) {
        securityPolicy = session._client._secureChannel.securityPolicy;
        assert(securityPolicy);
    }

    let identityToken;
    let serverCertificate = session.serverCertificate;
    // if server does not provide certificate use unencrypted password
    if (!serverCertificate) {
        identityToken = new UserNameIdentityToken({
            encryptionAlgorithm: null,
            password: Buffer.from(password, "utf-8"),
            policyId: userTokenPolicy ? userTokenPolicy.policyId : null,
            userName: userName,
        });
        return identityToken;
    }

function adjustSecurityPolicy(channel, userTokenPolicy_securityPolicyUri) {
    // check that userIdentityToken
    let securityPolicy = fromURI(userTokenPolicy_securityPolicyUri);

    // if the security policy is not specified we use the session security policy
    if (securityPolicy === SecurityPolicy.Invalid) {
        securityPolicy = fromURI(channel.clientSecurityHeader.securityPolicyUri);
        assert(securityPolicy);
    }
    return securityPolicy;
}