Add support for transfer transactions

As raised in https://discord.com/channels/905194001349627914/1235606151488602142

the [TransactionPrep](https://github.com/hyperledger/firefly-evmconnect/blob/67fdd4913465665a4e21d47636a5674323bdfcf3/internal/ethereum/prepare_transaction.go#L46) function doesn't handle native transfer in which `data` won't be provided.