I'm using the bitcoin rust crate to build my own CLI wallet.In my function where I'm constructing the TxIn's for my Transaction, is the actual script_sig required at the below step? Or does it populate when I go and sign the transaction in a later step?let txins: Vec = selected_utxos .iter() .map(|utxo| TxIn { previous_output: bitcoin::OutPoint { txid: bitcoin::Txid::from_str(&utxo.txid).expect("Invalid txid format"), vout: utxo.vout, }, script_sig: ScriptBuf::new(), sequence: Sequence::MAX, witness: Witness::new(), }) .collect();Is it ok to put ScriptBuf::new() as a placeholder?