Adjust value assignment logic in event.py

[M123-dev]

Fixes: #224

Somehow I (and some others) got a "Verkaufsorder"/Sale with the attribute

"amount": {
      "currency": "EUR",
      "value": 0.0,
      "fractionDigits": 2
    },

Which caused an error because the parser explicitly checked for value != 0.0.

This pull request removes the explicit check for zero values when parsing the amount.value field, so a value of 0.0 will now be considered valid instead of being replaced with None.

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That being said I am not sure why there was this explicit check, if it has a specific reason this PR should probably not be merged as it. I am using my fork right now, and it works for me 👍🏼

[RealCLanger]

Thanks for the contribution. I enabled the checks and it shows that there is a formatting error. Can you please take a look and update your change.

The parsing tests seem to work fine, though, so I think your change should be integrated when everything is green.