Fix: omit optional invoice contents from json sent to lexoffice

[rwos]

It would previously for example always send an empty paymentConditions { ... } thingy and then lexoffice complained that the paymentLabel is too short.

Not all of the omitempty fields are really optional (for example netAmount or grossAmount must be sent, depending on taxType) - but they shouldn't be sent with null-values when they are not set.