fix(sdk): correctly handle no batch case

Fixes the SDK so that it correctly handles the case where no batches
have been submitted to the StateCommitmentChain yet. Only happens in
development or immediately after a new chain is started.

.changeset/grumpy-ads-smile.md

+---
+'@eth-optimism/sdk': patch
+---
+
+Have SDK properly handle case when no batches are submitted yet

packages/sdk/src/cross-chain-messenger.ts

@@ -654,6 +654,11 @@ export class CrossChainMessenger implements ICrossChainMessenger {
     let batchEvent: ethers.Event | null =
       await this.getStateBatchAppendedEventByBatchIndex(upperBound)
 
+    // Only happens when no batches have been submitted yet.
+    if (batchEvent === null) {
+      return null
+    }
+
     if (isEventLo(batchEvent, transactionIndex)) {
       // Upper bound is too low, means this transaction doesn't have a corresponding state batch yet.
       return null