`TransactionEvent` can be `FormatViolation` without `triggerReason`

[KSR-Yasuda]

In some cases, TransactionEvent do not have the required property triggerReason.

In TransactionService::Evse::loop(), here it sets event triggerReason, but it may be the initial value UNDEFINED.

Then, the triggerReason property is skipped in message serialization.

Shouldn't you set something to follow the OCPP spec?