fix: correctly process chainId field (#370)

* fix: correctly process chainId field

* skip if none

* fix

crates/rpc-types/src/eth/transaction/mod.rs

@@ -68,7 +68,7 @@ pub struct Transaction {
     #[serde(flatten, skip_serializing_if = "Option::is_none")]
     pub signature: Option<Signature>,
     /// The chain id of the transaction, if any.
-    #[serde(with = "alloy_serde::u64_hex_opt")]
+    #[serde(default, skip_serializing_if = "Option::is_none", with = "alloy_serde::u64_hex_opt")]
     pub chain_id: Option<u64>,
     /// Contains the blob hashes for eip-4844 transactions.
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
@@ -314,4 +314,24 @@ mod tests {
         let deserialized: Transaction = serde_json::from_str(&serialized).unwrap();
         assert_eq!(transaction, deserialized);
     }
+
+    #[test]
+    fn serde_minimal_transaction() {
+        let transaction = Transaction {
+            hash: B256::with_last_byte(1),
+            nonce: 2,
+            from: Address::with_last_byte(6),
+            value: U256::from(8),
+            gas: U256::from(10),
+            input: Bytes::from(vec![11, 12, 13]),
+            ..Default::default()
+        };
+        let serialized = serde_json::to_string(&transaction).unwrap();
+        assert_eq!(
+            serialized,
+            r#"{"hash":"0x0000000000000000000000000000000000000000000000000000000000000001","nonce":"0x2","blockHash":null,"blockNumber":null,"transactionIndex":null,"from":"0x0000000000000000000000000000000000000006","to":null,"value":"0x8","gas":"0xa","input":"0x0b0c0d"}"#
+        );
+        let deserialized: Transaction = serde_json::from_str(&serialized).unwrap();
+        assert_eq!(transaction, deserialized);
+    }
 }