Bug 14004: Set the syspref's value to an empty string

Bug 14004: Set the syspref's value to an empty string

It's tested with "defined" in C4::Context->preference

Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 6b23c0634b43a60f0a434d4c43a3e57718ad0806)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit efdf8bb303d0fd37a80150f41193b34173e23d3f)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
(cherry picked from commit 1ec05c1c3225c9baaec2011831dc6f69fc94caac)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>