[zksend] Make contract optional in ZkSendLinkBuilder.createLinks (Mys…

…tenLabs#17013)

## Description 

Describe the changes or additions included in this PR.

## Test Plan 

How did you test the new or updated feature?

---
If your changes are not user-facing and do not break anything, you can
skip the following section. Otherwise, please briefly describe what has
changed under the Release Notes section.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

.changeset/giant-eyes-juggle.md

@@ -0,0 +1,5 @@
+---
+'@mysten/zksend': patch
+---
+
+Make contract optional in ZkLSendLinkBuilder.createLinks

sdk/zksend/src/links/builder.ts

@@ -295,7 +295,7 @@ export class ZkSendLinkBuilder {
 		client?: SuiClient;
 		network?: 'mainnet' | 'testnet';
 		links: ZkSendLinkBuilder[];
-		contract: ZkBagContractOptions;
+		contract?: ZkBagContractOptions;
 	}) {
 		const contract = new ZkBag(contractIds.packageId, contractIds);
 		const store = transactionBlock.object(contract.ids.bagStoreId);