TransasctionEvent req w/o meterValues fails to be FormationViolation

[KSR-Yasuda]

TransactionEvent request with no meterValues property fails to be replied FormationViolation.

// Request to OCPP.Core
// Has no `meterValues` prop.
[
    2,
    "OCPP_MSG_3735759508",
    "TransactionEvent",
    {
        "eventType": "Started",
        "timestamp": "2022-12-31T23:00:06",
        "triggerReason": "CablePluggedIn",
        "seqNo": 6270,
        "transactionInfo": {
            "transactionId": "test-id"
        }
    }
]

// Response from OCPP.Core
[
    4,
    "OCPP_MSG_3735759508",
    "FormationViolation",
    "",
    {}
]

// `OCPP.Core.Server.dll` Log

info: OCPPMiddleware.OCPP20[0]
      OCPPMiddleware.Receive20 => OCPP-Message: Type=2 / ID=OCPP_MSG_3735759508 / Action=TransactionEvent)

fail: OCPP.Core.Server.ControllerOCPP20[0]
      TransactionEvent => Exception: Object reference not set to an instance of an object.
      System.NullReferenceException: Object reference not set to an instance of an object.
         at OCPP.Core.Server.ControllerOCPP20.GetMeterValues(ICollection`1 meterValues, Double& meterKWH, Double& currentChargeKW, Double& stateOfCharge, Nullable`1& meterTime) in /xxxxxx/OCPP.Core/OCPP.Core.Server/ControllerOCPP20.TransactionEvent.cs:line 331
         at OCPP.Core.Server.ControllerOCPP20.HandleTransactionEvent(OCPPMessage msgIn, OCPPMessage msgOut) in /xxxxxx/OCPP.Core/OCPP.Core.Server/ControllerOCPP20.TransactionEvent.cs:line 60

---

meterValues prop of TransactionEvent req is optional, however, without it,
it fails in GetMeterValues() in foreach for meterValues of null-reference.

Besides, ahead of there, it can fail if ChargeTag ct is null, too.

At least, these two parts should be null-value considered.