Remove Some null handling wart

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Some null renders as null atm; as evidenced by the diffs, removing that wart makes for less special cases in tests re #38

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@ylibrach any insights much appreciated ;)

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Hm; conditional serialization is not a thing in System.Text.Json v 4.7; there's no reasonable way to implement such a special case
Also, going down the rathole has removed any desire I have to special case it - I've simply noted that Some null is not a roundtrippable value.