Bug 29204: Fix reports/issues_stats.pl for MySQL 8

Bug 29204: Fix reports/issues_stats.pl for MySQL 8

Error was: Incorrect DATETIME value: ''

Test plan
Go to Reports -> Circulation
On Period select "Row" or "Column" and choose a date
Submit

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 9a4b7b1948ecba51c942bdbb6b202981a6cce144)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>