Bug 36939: Remove a warning from Serials.t

Bug 36939: Remove a warning from Serials.t

t/db_dependent/Serials.t .. 2/57 Use of uninitialized value in numeric gt (>) at /kohadevbox/koha/C4/Serials.pm line 2029.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 7d75ec3e2602dcadbb238dfb7cbec76020ebafef)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit f41e7398857cc9f76c0bcdf01e3426c192e773e0)
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>