Bug 28744: Regression tests

Bug 28744: Regression tests

This patch adds tests for the following cases:
- to_api_mapping not defined
- to_api_mapping returning an empty hashref

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Object.t
=> FAIL: Tests fail :-(

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
JD amended patch: remove uneeded commented lines

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>