Fix precision on the calculation of the backlog cost and the l1 gas p… (#3777)

* Fix precision on the calculation of the backlog cost and the l1 gas price in the case of blobs

* Avoid rounding issues and nil pointer in l1 gas price calculation

* Update execution/gethexec/sequencer.go

* Avoid duplicate calculations and solve rounding problem

---------

Co-authored-by: Joshua Colvin <[email protected]>

Author: alexandrosfilios

execution/gethexec/sequencer.go

@@ -1432,10 +1432,20 @@ func (s *Sequencer) updateExpectedSurplus(ctx context.Context) (int64, error) {
 			if err != nil {
 				return 0, fmt.Errorf("error encountered getting blob base fee while updating expectedSurplus: %w", err)
 			}
-			blobFeePerByte.Mul(blobFeePerByte, blobTxBlobGasPerBlob)
-			blobFeePerByte.Div(blobFeePerByte, usableBytesInBlob)
-			l1GasPrice = blobFeePerByte.Int64() / 16
-			backlogCost = (backlogCallDataUnits * blobFeePerByte.Int64()) / 16
+
+			if backlogCallDataUnits == 0 {
+				blobFeePerByte.Mul(blobFeePerByte, blobTxBlobGasPerBlob)
+				blobFeePerByte.Div(blobFeePerByte, usableBytesInBlob)
+				l1GasPrice = blobFeePerByte.Int64() / 16
+				backlogCost = 0
+			} else {
+				// l1GasPrice can be zero because of roundings, hence backlogCost is calculated separately
+				backlogFee := big.NewInt(backlogCallDataUnits)
+				backlogFee.Mul(backlogFee, blobTxBlobGasPerBlob)
+				backlogFee.Div(backlogFee, usableBytesInBlob)
+				backlogCost = backlogFee.Int64() / 16
+				l1GasPrice = backlogCost / backlogCallDataUnits
+			}
 		}
 	case "CalldataPrice7623":
 		l1GasPrice = (header.BaseFee.Int64() * 40) / 16