fix: fpac sdk bug (#9367)

Fixes a small bug in the SDK that could cause an error if there
aren't any games yet.

.changeset/modern-owls-rescue.md

+---
+'@eth-optimism/sdk': patch
+---
+
+Fixes a bug in the SDK for finalizing fpac withdrawals.

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

@@ -1275,7 +1275,7 @@ export class CrossChainMessenger {
       const latestGames =
         await this.contracts.l1.DisputeGameFactory.findLatestGames(
           gameType,
-          gameCount.sub(1),
+          Math.max(0, gameCount.sub(1).toNumber()),
           Math.min(100, gameCount.toNumber())
         )