Bug 18403: Fix error triggered by subgroups being added to group

Bug 18403: Fix error triggered by subgroups being added to group

With one root group and two child groups and it works fine.
But if I create any groups under *those* groups, not only does it not work,
I get the following error:
Template process failed: undef error - SQL::Abstract::puke(): [SQL::Abstract::__ANON__]
Fatal: SQL::Abstract before v1.75 used to generate incorrect SQL when the -IN operator was
given an undef-containing list: !!!AUDIT YOUR CODE AND DATA!!! (the
upcoming Data::Query-based version of SQL::Abstract will
emit the logically correct SQL instead of raising this
exception) at /home/vagrant/kohaclone/Koha/Objects.pm line 269

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