Fix typecheck: use undefined instead of null for OnyxEntry parameter

OnyxEntry<T> is T | undefined (not T | null), so passing null to
setSplitShares caused TS2345.

Co-authored-by: Eskalifer1 <Eskalifer1@users.noreply.github.com>

Author: MelvinBot

tests/actions/IOU/IOUSettersTest.ts

@@ -302,7 +302,7 @@ describe('IOU setter functions', () => {
         });
 
         it('should do nothing when transaction is null', async () => {
-            setSplitShares(null, 10000, 'USD', [USER_ACCOUNT_ID, PARTICIPANT_ACCOUNT_ID]);
+            setSplitShares(undefined, 10000, 'USD', [USER_ACCOUNT_ID, PARTICIPANT_ACCOUNT_ID]);
             await waitForBatchedUpdates();
 
             const draft = await getOnyxValue(`${ONYXKEYS.COLLECTION.TRANSACTION_DRAFT}${TRANSACTION_ID}`);