Bug 29804: Fix Koha::Hold->is_pickup_location_valid exploding

Bug 29804: Fix Koha::Hold->is_pickup_location_valid exploding

This trivial patch acknowledges the fact bug 28871 is probably not going
to be pushed, and changes the method so  it, internally, uses
Koha::Item->pickup_locations and Koha::Biblio->pickup_locations in
scalar context.

This is probably the correct solution as the discussion on bug 28883
points towards the future removal of the use of wantarray altogether,
eventually.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Hold.t
=> FAIL: It explodes as we see in production, with:
The method Koha::Libraries->branchcode is not covered by tests!
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: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>