[zksend] fix how coin balances are merged (MystenLabs#15692)

## 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/brown-buttons-accept.md

@@ -0,0 +1,5 @@
+---
+'@mysten/zksend': patch
+---
+
+Fix coin merging for sending balances

sdk/zksend/src/links.ts

@@ -137,9 +137,11 @@ export class ZkSendLinkBuilder {
 				objectsToTransfer.push(sui);
 			} else {
 				const coins = (await this.#getCoinsByType(coinType)).map((coin) => coin.coinObjectId);
-				const merged =
-					coins.length > 1 ? txb.mergeCoins(coins[0], coins.slice(1)) : txb.object(coins[0]);
-				const [split] = txb.splitCoins(merged, [amount]);
+
+				if (coins.length > 1) {
+					txb.mergeCoins(coins[0], coins.slice(1));
+				}
+				const [split] = txb.splitCoins(coins[0], [amount]);
 				objectsToTransfer.push(split);
 			}
 		}