Bug 7806: Fix remaining occurrences of 0000-00-00

Bug 7806: Fix remaining occurrences of 0000-00-00

We should remove all SQL queries that contain 0000-00-00 and finally
assume we do not longer have such value in our DB (for date type)

We already dealt with such values in previous update DB entries.
The 2 added by this one haven't been replaced already.

The code will now assume that either a valid date exist, or NULL/undef.

Test plan:
QA review is needed and test of the different places where code is
modified.

Not sure about the change from reports/issues_avg_stats.pl

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>