Bug 32800: Don't embed items for deleted records

Bug 32800: Don't embed items for deleted records

To test:
1 - Delete a biblio
2 - perl /kohadevbox/koha/misc/migration_tools/build_oai_sets.pl -v -i -r
3 - Error:
    Can't call method "items" on an undefined value at /kohadevbox/koha/Koha/Biblio/Metadata.pm line 163.
4 - Apply patch
5 - Repeat
6 - Success!

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Works as advertised.
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>