Initialize the collection

[vanillajonathan]

https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/guidelines-for-collections

DO NOT return null values from collection properties or from methods returning collections. Return an empty collection or an empty array instead.

DO NOT use ArrayList or List<T> in public APIs.

[richardrandak]

Hello! Thanks for the pull request. However, if we do this change, we should change it for all connectors, not just voucher, to keep keep it consistent.

[vanillajonathan]

Yes, I think all collections should be initialized to empty collections, this makes it easier to use and you can iterate over them without worrying about null values.

[richardrandak]

I see one issue with initialized collections. Current SDK behaviour is that only populated properties are serialized when UPDATE request is called. If the collection is auto-initialized, that would mean, the collection property would be populated (with empty collection) and therefore, would be updated to empty one without user's knowledge