[indexer.api.add_withdrawal_hash] verify message hash not nil

.vscode/launch.json

+{
+  // Use IntelliSense to learn about possible attributes.
+  // Hover to view descriptions of existing attributes.
+  // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+  "version": "0.2.0",
+  "configurations": [
+      {
+
+          "type": "go",
+          "request": "launch",
+          "name": "Launch Program",
+          "program": "${workspaceFolder}/indexer/cmd/indexer",
+          "mode": "debug",
+          "env": {
+              "INDEXER_CONFIG": "/Users/ethen.pociask@coinbase.com/coinbase/indexer.sepolia.toml",
+          },
+          "args": [
+              "index"
+          ]
+      }
+  ]
+}

indexer/api/models/models.go

 package models
 
-import "github.com/ethereum-optimism/optimism/indexer/database"
+import (
+	"github.com/ethereum-optimism/optimism/indexer/database"
+	"github.com/ethereum/go-ethereum/common"
+)
 
 // DepositItem ... Deposit item model for API responses
 type DepositItem struct {
@@ -51,6 +54,12 @@ type WithdrawalResponse struct {
 func CreateWithdrawalResponse(withdrawals *database.L2BridgeWithdrawalsResponse) WithdrawalResponse {
 	items := make([]WithdrawalItem, len(withdrawals.Withdrawals))
 	for i, withdrawal := range withdrawals.Withdrawals {
+
+		msg := withdrawal.L2BridgeWithdrawal.CrossDomainMessageHash
+		if msg == nil {
+			msg = &common.Hash{}
+		}
+
 		item := WithdrawalItem{
 			Guid:                 withdrawal.L2BridgeWithdrawal.TransactionWithdrawalHash.String(),
 			L2BlockHash:          withdrawal.L2BlockHash.String(),
@@ -59,7 +68,7 @@ func CreateWithdrawalResponse(withdrawals *database.L2BridgeWithdrawalsResponse)
 			To:                   withdrawal.L2BridgeWithdrawal.Tx.ToAddress.String(),
 			TransactionHash:      withdrawal.L2TransactionHash.String(),
 			Amount:               withdrawal.L2BridgeWithdrawal.Tx.Amount.String(),
-			MessageHash:          withdrawal.L2BridgeWithdrawal.CrossDomainMessageHash.String(),
+			MessageHash:          msg.String(),
 			ProofTransactionHash: withdrawal.ProvenL1TransactionHash.String(),
 			ClaimTransactionHash: withdrawal.FinalizedL1TransactionHash.String(),
 			L1TokenAddress:       withdrawal.L2BridgeWithdrawal.TokenPair.RemoteTokenAddress.String(),