Bug 35979: (follow-up) Add check in ->enqueue

Bug 35979: (follow-up) Add check in ->enqueue

Foundation for removing same check elsewhere (later on).

Test plan:
Run t/db_dependent/Koha/BackgroundJob/BatchUpdateBiblioHoldsQueue.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
PA amended: Fix bug # in commit message

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit c317bb34c0bb59fd0f52b4e7c5c0d3326b0030ee)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>