Update ZKInputs for empty batches

- Add TestZKInputsEmpty circom test vectors
- Fix ZKInputsHash OldLastIdx bytearray

common/zk.go

@@ -431,8 +431,9 @@ func (z ZKInputs) ToHashGlobalData() ([]byte, error) {
 
 	// [MAX_NLEVELS bits] oldLastIdx
 	oldLastIdx := make([]byte, bytesMaxLevels)
-	copy(oldLastIdx, z.OldLastIdx.Bytes())
-	b = append(b, SwapEndianness(oldLastIdx)...)
+	oldLastIdxBytes := z.OldLastIdx.Bytes()
+	copy(oldLastIdx[len(oldLastIdx)-len(oldLastIdxBytes):], oldLastIdxBytes)
+	b = append(b, oldLastIdx...)
 
 	// [MAX_NLEVELS bits] newLastIdx
 	newLastIdx := make([]byte, bytesMaxLevels)

db/statedb/txprocessors.go

@@ -96,6 +96,7 @@ func (s *StateDB) ProcessTxs(ptc ProcessTxsConfig, coordIdxs []common.Idx, l1use
 		s.zki = common.NewZKInputs(ptc.MaxTx, ptc.MaxL1Tx, ptc.MaxTx, ptc.MaxFeeTx, ptc.NLevels, s.currentBatch.BigInt())
 		s.zki.OldLastIdx = s.idx.BigInt()
 		s.zki.OldStateRoot = s.mt.Root().BigInt()
+		s.zki.Metadata.NewLastIdxRaw = s.idx
 	}
 
 	// TBD if ExitTree is only in memory or stored in disk, for the moment