fix: return 204 when UE context missing and avoid nil deref

[wiwi878]

This PR is for issue#781

[c9274326]

Hi @wiwi878,

It seems like the functional test part is currently stuck.
Please execute ./test.sh TestEAPAKAPrimeAuthentication and check the logs?

Please take a look at the failure, thanks.

2026-02-12T12:05:57.630963826Z [INFO][AMF][Gmm][amf_ue_ngap_id:RU:1,AU:1(3GPP)][supi:SUPI:] Start T3560 timer
2026-02-12T12:05:57.631048996Z [INFO][AMF][Ngap][ran_addr:127.0.0.1:9487] Handle UplinkNASTransport
2026-02-12T12:05:57.631073772Z [INFO][AMF][Ngap][amf_ue_ngap_id:RU:1,AU:1(3GPP)][ran_addr:127.0.0.1:9487] Handle UplinkNASTransport (RAN UE NGAP ID: 1)
2026-02-12T12:05:57.631190434Z [INFO][AMF][Gmm] Handle event[Gmm Message], transition from [Authentication] to [Authentication]
2026-02-12T12:05:57.631280137Z [INFO][AMF][Gmm][amf_ue_ngap_id:RU:1,AU:1(3GPP)][supi:SUPI:] Handle Authentication Response
2026-02-12T12:05:57.631340723Z [INFO][AMF][Gmm][amf_ue_ngap_id:RU:1,AU:1(3GPP)][supi:SUPI:] Stop T3560 timer
2026-02-12T12:05:57.632720375Z [INFO][AUSF][5gAka] EapAuthComfirmRequest
2026-02-12T12:05:57.632938467Z [INFO][AUSF][Eap] Correct RES value, EAP-AKA' auth succeed
2026-02-12T12:05:57.634024782Z [WARN][UDM][UEAU] Supi is invalid
2026-02-12T12:05:57.634749529Z [INFO][UDM][GIN] | 400 |       127.0.0.1 | POST    | /nudm-ueau/v1/suci-0-208-93-0-0-0-0000007487/auth-events |  |
2026-02-12T12:05:57.637430323Z [ERRO][AUSF][Consumer] Error in ConfirmAuth: 400, Bad Request
2026-02-12T12:05:57.637519864Z [INFO][AUSF][Eap] 400, Bad Request
2026-02-12T12:05:57.642372844Z [INFO][AUSF][GIN] | 500 |       127.0.0.1 | POST    | /nausf-auth/v1/ue-authentications/suci-0-208-93-0-0-0-0000007487/eap-session |  |
2026-02-12T12:05:57.642882778Z [ERRO][AMF][Gmm] 500, Internal Server Error