MySQL strict mode rejects '' for a DATE/DATETIME column, so any form
with an untouched optional date aborted the whole insert with
"Incorrect date value: ''". This was not one bug — quote responses,
vouchers, bounced cheques, bank loans, documentary credits, letters of
guarantee, settlements, tenders, auctions, committees, payments,
overtime, permission requests and more all passed the raw post value
straight through.

Adds Request::postDate(), which returns NULL for an empty date field,
and routes all 33 optional-date call sites through it. QuoteService
also guards expiry_date directly since it is called from more than one
path.