Bug 36891: Restore returning 404 from svc/bib when the bib number doesn't exist

Bug 36891: Restore returning 404 from svc/bib when the bib number doesn't exist

Changing from GetMarcBiblio to Biblios->find plus metadata->record lost the
way that svc/bib used to return 404 when the bib number wasn't found. This
patch restores that by checking for undef after the Biblios->find step.

Test plan:
1. Load e.g. http://127.0.0.1:8081/cgi-bin/koha/svc/bib/289 which returns an
   XML bib record
2. Load http://127.0.0.1:8081/cgi-bin/koha/svc/bib/99999999 and get a 500 error
2. Appply patch, restart_all
4. Reload http://127.0.0.1:8081/cgi-bin/koha/svc/bib/289 and get the bib again
5. Reload http://127.0.0.1:8081/cgi-bin/koha/svc/bib/99999999 and get a 404

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>