Bug 28744: Better handling of undefined to_api_mapping

Bug 28744: Better handling of undefined to_api_mapping

We always had some mapping because of the DB vs. API object definition discrepancies. But bug 25260 revamps the reserves table, and it is a perfect match. It highlights this edge case: if no mapping defined, then and undef from_api_mapping is generated (as opposed to an empty hashref) and this leads to errors in the query translation from the API.

This patch makes a small change so this method always returns an empty
hashref.

To test:
1. Apply the regression tests
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Object.t
=> FAIL: Test fail!
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

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>