ThankYou Date is Optional

If thankyou_date is already set in a contribution ThankYou_send will not
send an email. In some cases we do not want to send an email, e.g. we got
a check; so we have to set this to something. This allows both cases.

Change-Id: Ie21bd84b1373836277046d8b6e2cdd74cad5125a