Bug 15090: Fix date filter for Suggestions management

Bug 15090: Fix date filter for Suggestions management

Home > Acquisitions > Suggestions management:
Before 13813 the "from" date in filters was 0000-00-00 if not
filled, afer 13813 it's today.

This patch re-introduces 0000-00-00.

See http://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=13813#c172

To test:
- Go to Home > Acquisitions > Suggestions management
- Verify that date filters work as expected
- Verify that start dates in sql query are 0000-00-00 if no start
  dates are given
- Note: Line 185 outputs the start dates as warn if $debug is set.

Signed-off-by: Nicole Engard <nengard@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>