Bug 23867: Resolve "truncated incorrect double" for empty string

Bug 23867: Resolve "truncated incorrect double" for empty string

Updatedatabase, rev. 18.12.00.051
Solved by testing the value of the pref first.
Tested by copying the change in a new atomic update with XXX.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 6d1b3cf2120fbd2c9beb4317797c260d9b647f79)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>