redirect irule when an client certificate is missing

Your iRule fails because the variable

$subject_dn

when RULE_INIT {
    set static::debug 1
}

when CLIENTSSL_CLIENTCERT {
    set subject_dn [X509::subject [SSL::cert 0]] 
    if { $subject_dn != "" }{
        if { $static::debug }{ log "Client Certificate received: $subject_dn"}
    }
}

when CLIENTSSL_HANDSHAKE {
    if { [SSL::verify_result] == !0 } then {set clientCRT 1} else {set clientCRT 0}
}

when HTTP_REQUEST {
    if {[info exists subject_dn]} {
        switch -glob -- $subject_dn {
            "*CN=Vinit-A*" {HTTP::redirect "/vinit-A.html"}
            "*CN=Vinit-B*" {HTTP::redirect "/vinit-B.html"}
            "*CN=Vinit-B*" {HTTP::redirect "/vinit-C.html"}
        }
    }
    else {
        HTTP::redirect "/Certmissing.html"
    }
}