Bug 26997: CAST AS CHAR to prevent the error

Bug 26997: CAST AS CHAR to prevent the error

To prevent the DBMS to fail on 0000-00-00 we can cast it as a char and
compare.
This patch also moves to a subroutine, in case we need to adjust (or
reuse) it later.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 0a40b4bb509c306794930ef734513a982e418af5)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 9327130853613d5518326fddcabb1ef153daacbd)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>