cbor: omit null map entries for Option::None

[karalabe]

OpenAI Codex 5.3 via OpenCode. 3:30 min, 42,088 tokens.

Our current CBOR encoder encodes Option<> None as cbor::Null. We only ever used it in array positionals so that's fine. But in maps, this doesn't make sense, we should just omit the field. Lets plan this out , let me confirm, then we can implement it