Bug 29306: Fix warning on undefined $_ in Holds.t

Bug 29306: Fix warning on undefined $_ in Holds.t

Use of uninitialized value $_ in concatenation (.) or string at t/db_dependent/Holds.t line 1658.

Results from copying line 70.

Test plan:
Run Holds.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 531f710cacb848a1d7a75f182fb8f9fe9b8c9c89)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>