fix(accounting): voucher save crashed when payment method is not a cheque

The cheque fields stay in the voucher form for every payment method, so
check_date posts as '' when paying cash or by transfer. That went
straight into a DATE column and MySQL rejected the row with
"Incorrect date value: ''", so no non-cheque voucher could be saved at
all. Send NULL instead; VoucherService already accepted null.

Found by driving the voucher screen end-to-end while building the
step-by-step accounting tutorial.

app/Modules/Accounting/Controllers/VoucherController.php

@@ -111,7 +111,9 @@ class VoucherController extends Controller
             'member_id'          => $request->post('member_id'),
             'check_number'       => $request->post('check_number'),
             'check_bank'         => $request->post('check_bank'),
-            'check_date'         => $request->post('check_date'),
+            // The cheque fields stay in the form even when paying cash/bank, so they
+            // arrive as '' — and '' is not a DATE. Send NULL or MySQL rejects the row.
+            'check_date'         => trim((string) $request->post('check_date', '')) ?: null,
             'reference'          => $request->post('reference'),
             'description_ar'     => $request->post('description_ar'),
             'notes'              => $request->post('notes'),