-refac: check if transcation is coinbase transaction before hashing

- refac: represent witness reserved value in string literal

packages/consensus/src/validation/block.cairo

@@ -46,13 +46,13 @@ pub fn compute_and_validate_tx_data(
         let tx_bytes_segwit = @tx.encode_with_witness(tx_bytes_legacy);
 
         let txid = double_sha256_byte_array(tx_bytes_legacy);
-        let mut wtxid = double_sha256_byte_array(tx_bytes_segwit);
 
         /// The wTXID for the coinbase transaction must be set to all zeros. This is because it's
         /// eventually going to contain the commitment inside it
         /// see https://learnmeabitcoin.com/technical/transaction/wtxid/#commitment
-        if i == 0 {
-            wtxid = Default::default();
+        let mut wtxid = Default::default();
+        if i != 0 {
+            wtxid = double_sha256_byte_array(tx_bytes_segwit);
         }
 
         // tx_byte_segwit represents all the bytes in the transaction, so the bytes in the segwit

packages/consensus/src/validation/coinbase.cairo

@@ -7,12 +7,10 @@ use utils::{
     bit_shifts::shr, hash::{Digest, DigestIntoByteArray}, sha256::{double_sha256_byte_array}
 };
 
-
 const BIP_34_BLOCK_HEIGHT: u32 = 227_836;
 const BIP_141_BLOCK_HEIGHT: u32 = 481_824;
 const WTNS_PK_SCRIPT_LEN: u32 = 38;
 const WTNS_PK_SCRIPT_PREFIX: felt252 = 116705705699821; // 0x6a24aa21a9ed
-const WITNESS_VALUE: felt252 = 0;
 
 /// Validates coinbase transaction.
 pub fn validate_coinbase(
@@ -41,8 +39,8 @@ pub fn validate_coinbase(
         let witness = tx.inputs[0].witness[0];
 
         // check witness value
-        let mut witness_value_byte: ByteArray = "";
-        witness_value_byte.append_word(WITNESS_VALUE, 32);
+        let witness_value_byte: ByteArray =
+            "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
 
         if witness != @witness_value_byte {
             return Result::Err("Wrong coinbase witness");
@@ -134,8 +132,8 @@ fn compute_block_reward(block_height: u32) -> u64 {
 fn calculate_wtxid_commitment(wtxid_root: Digest) -> Digest {
     // construct witness reserved value
     // 0000000000000000000000000000000000000000000000000000000000000000
-    let mut witness_value_byte: ByteArray = "";
-    witness_value_byte.append_word(WITNESS_VALUE, 32);
+    let witness_value_byte: ByteArray =
+        "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
 
     // convert wtxid_root to ByteArray
     let wtxid_root_bytes: ByteArray = wtxid_root.into();