Bug 28472: (follow-up) add unit test for the case where location = NULL

Bug 28472: (follow-up) add unit test for the case where location = NULL

This patch adds an extra unit test, to cover the case where the
UpdateItemLocationOnCheckin System Preference is set to "_ALL_: CART"
and the item being returned has no shelving location set.

Test plan:

1) Apply the patch provided earlier
2) prove -v t/db_dependent/Circulation/issue.t
   ...and sign off if all tests pass.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
JD amended patch: use 'is' instead of 'ok'

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