transactionId=0 being synonymous with "null" in MeterValues

[goekay]

Checklist

• I checked other issues already and found no answer
• I checked the documentation and found no answer
• I am running the latest version and the feature i am requesting is not implemented there

Describe the problem you are trying to solve

In MeterValues messages, some charging stations seem to set transactionId=0 to communicate that there is no transaction reference. Actually, transactionId is nullable and therefore this attribute can be omitted from payload.

This is a charging station misbehaviour. But it can be fixed (worked around) easily without harming the integrity of the business flows. Our valid transaction ids are coming from a database sequence (PK) and start with 1 anyways. So, there is no overlap between the value ranges.

Describe the solution you'd like

Treat transactionId=0 the same as transactionId=null

Describe alternative solutions or features you've considered

Leave it as is. But we cannot process these messages and are dropping them causing information loss.

Additional context

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