Bug 28992: Resolve warning on BIG_LOOP

Bug 28992: Resolve warning on BIG_LOOP

Argument "" isn't numeric in numeric gt (>) at
/usr/share/koha/koha-tmpl/intranet-tmpl/prog/en/modules/cataloguing/
addbiblio.tt line 896.

Came across it when testing 28608. Could have been a QA follow-up.

Test plan:
If you run QA tools on this patch, you should see the warning only
when processing files before patches.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>