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.