TransactionRequest loses contract creation when deserializing `to: null`

[1sgtpepper]

Summary

TransactionRequest::to uses an outer Option<TxKind>. A creation request represented as Some(TxKind::Create) serializes to the JSON/RPC form to: null, but deserializing that JSON produces to == None.

This collapses the distinction between a known contract creation and an incomplete request without a destination.

Reproduction

let request = TransactionRequest::default().create();
let json = serde_json::to_value(&request).unwrap();
let decoded: TransactionRequest = serde_json::from_value(json).unwrap();

assert_eq!(decoded.to, Some(TxKind::Create));

The assertion currently fails because decoded.to is None.

Expected JSON/RPC behavior

• omitted to deserializes as None;
• present to: null deserializes as Some(TxKind::Create);
• present address deserializes as Some(TxKind::Call(address)).

The binary bincode format must retain its existing decoding behavior.

Related

#4071 correctly preserves TxKind::Create when converting concrete consensus transactions into TransactionRequest and emits to: null. This issue tracks the pre-existing inbound JSON/RPC round-trip loss separately to keep that PR scoped.