Bug 32302: Hide "ISBN" label when no ISBN data when sending list

Bug 32302: Hide "ISBN" label when no ISBN data when sending list

There is a logic mistake in the opac-sendshelf.tt template where
we test if the "ISBN" key exists in the "BIBLIO_RESULT" hash when
we should really be testing if the arrayref returned by "ISBN" is
empty or not, since that "ISBN" key always exists and contains at least
an empty arrayref.

Test plan:
1. Apply patch and restart your Koha Starman
2. Create a public list containing records with 020 MARC fields and
records without 020 MARC fields
3. View the list in the OPAC
4. Click "Send list" and email it to yourself
5. Note that only records with 020 MARC fields with show "ISBN:"
in the email output

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>