Bug 35833: Fix warnings from C4/Koha

Bug 35833: Fix warnings from C4/Koha

[2024/01/18 11:52:40] [WARN] Argument " " isn't numeric in numeric eq (==) at /usr/share/koha/C4/Koha.pm line 600.
[2024/01/18 11:52:40] [WARN] Argument " " isn't numeric in numeric eq (==) at /usr/share/koha/C4/Koha.pm line 659.

Note: According to the code in MARC::Field the indicator returned cannot be undef. So we do not return new
uninitialized warnings.

Test plan:
Check your log before and after patch.
Possibly you may need to create a record with spaces as field indicators.

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: Katrin Fischer <katrin.fischer@bsz-bw.de>